Presence.Type有七种
  available （默认）用户空闲状态，即在线
  unavailable 用户没空看消息，即离线
  subscribe 请求订阅别人，即请求加对方为好友
  subscribed  统一被别人订阅，也就是确认被对方加为好友
  unsubscribe  他取消订阅别人，请求删除某好友
  unsubscribed  拒绝被别人订阅，即拒绝对方的添加请求
  error -- 当前状态packet有错误


好友双方对彼此的状态，有网友总结如下：
  none：是用户和自己roster中的好友彼此不关心，既不想把自己的presence状态告诉对方，也不愿意收到对方presence更新消息
  to：知道别人的出席消息，而别人不知道自己的出席信息
  from：别人知道自己的出席信息, 但自己不知道别人的出席信息
  Both:即收取对方状态更新，又将自己的更新告知对方
  Remove :将对方干掉，不再关心他的任何信息。

zxl1添加zxl2,加到Friends组，备注zxl2，C:client S:server
zxl1向zxl2发送添加请求Presence.Type.subscribe 订阅关系：none
zxl1->C:
    <iq id="zR0f2-29" type="set">
        <query xmlns="jabber:iq:roster">
            <item jid="zxl2@user-20160421db" name="zxl2">
                <group>Friends</group>
            </item>
        </query>
    </iq>
 S:
    <iq type="set" id="632-3020" to="zxl1@user-20160421db/Spark">
        <query xmlns="jabber:iq:roster">
            <item jid="zxl2@user-20160421db" name="zxl2" subscription="none">
                <group>Friends</group>
            </item>
        </query>
    </iq>
 
 zxl1->C:TCP
 S:<iq type="result" id="zR0f2-29" to="zxl1@user-20160421db/Spark"/>
 zxl1-> C:
        <presence id="zR0f2-30" to="zxl2@user-20160421db" type="subscribe">
            <x xmlns="vcard-temp:x:update">
                <photo>ba8c94a333af7a23d9e44c41b2aa22dd2224dadc</photo>
            </x>
            <x xmlns="jabber:x:avatar">
                <hash>ba8c94a333af7a23d9e44c41b2aa22dd2224dadc</hash>
            </x>
        </presence> 
 S:
    <iq type="set" id="331-3022" to="zxl1@user-20160421db/Spark">
        <query xmlns="jabber:iq:roster">
            <item jid="zxl2@user-20160421db" name="zxl2" ask="subscribe" subscription="none">
                <group>Friends</group>
            </item>
        </query>
    </iq> 
zxl1->C:TCP
之后就分为3个场景
场景一：zxl2收到zxl1的请求，拒绝zxl1 订阅关系：none
 S:
 <presence id="SIuGQ-28" to="zxl1@user-20160421db" type="unsubscribe">
    <x xmlns="vcard-temp:x:update">
        <photo>a76bac95d8fe05fb3df6142bc5cac14ce660345d</photo>
    </x>
    <x xmlns="jabber:x:avatar">
        <hash>a76bac95d8fe05fb3df6142bc5cac14ce660345d</hash>
    </x>
 </presence>
场景二：zxl2收到zxl1的请求，zxl2答应，zxl1拒绝 
1.zxl2收到zxl1的请求，zxl2答应 订阅关系：zxl1:from zxl2:to 
 C:    <iq id="A7O3Y-27" type="get">
        <ping xmlns=‘urn:xmpp:ping‘ />
    </iq>
 S:    <iq type="result" id="A7O3Y-27" to="zxl2@user-20160421db/Spark"/>
 C:    TCP
 C:    <iq id="A7O3Y-28" type="set">
        <query xmlns="jabber:iq:roster">
            <item jid="zxl1@user-20160421db" name="">
                <group>Friends</group>
            </item>
        </query>
    </iq>
 S:    <iq type="set" id="971-3086" to="zxl2@user-20160421db/Spark">
        <query xmlns="jabber:iq:roster">
            <item jid="zxl1@user-20160421db" name="" subscription="from">
                <group>Friends</group>
            </item>
        </query>
    </iq>
 C:    TCP
 S:    <iq type="result" id="A7O3Y-28" to="zxl2@user-20160421db/Spark"/>
 C:    <presence id="A7O3Y-29" to="zxl1@user-20160421db" type="subscribe">
        <x xmlns="vcard-temp:x:update">
            <photo>a76bac95d8fe05fb3df6142bc5cac14ce660345d</photo>
        </x>
        <x xmlns="jabber:x:avatar">
            <hash>a76bac95d8fe05fb3df6142bc5cac14ce660345d</hash>
        </x>
    </presence>
 S:    <iq type="set" id="505-3088" to="zxl2@user-20160421db/Spark">
        <query xmlns="jabber:iq:roster">
            <item jid="zxl1@user-20160421db" name="" ask="subscribe" subscription="from">
                <group>Friends</group>
            </item>
        </query>
    </iq>
 C:    <presence id="A7O3Y-30" to="zxl1@user-20160421db" type="subscribed">
        <x xmlns="vcard-temp:x:update">
            <photo>a76bac95d8fe05fb3df6142bc5cac14ce660345d</photo>
        </x>
        <x xmlns="jabber:x:avatar">
            <hash>a76bac95d8fe05fb3df6142bc5cac14ce660345d</hash>
        </x>
    </presence>
 S:    TCP
2.zxl1添加zxl2,zxl2答应，zxl1拒绝 zxl1:to zxl2:from
 C:     <presence id="RIdep-27" to="zxl2@user-20160421db" type="unsubscribe">
        <x xmlns="vcard-temp:x:update">
            <photo>ba8c94a333af7a23d9e44c41b2aa22dd2224dadc</photo>
        </x>
        <x xmlns="jabber:x:avatar">
            <hash>ba8c94a333af7a23d9e44c41b2aa22dd2224dadc</hash>
        </x>
    </presence>
 S:    <iq type="set" id="278-3097" to="zxl1@user-20160421db/Spark">
        <query xmlns="jabber:iq:roster">
            <item jid="zxl2@user-20160421db" name="zxl2" ask="unsubscribe" subscription="from">
                <group>Friends</group>
            </item>
        </query>
    </iq>
 C:    TCP
 C:    <iq id="RIdep-28" type="get"><ping xmlns=‘urn:xmpp:ping‘ /></iq>
 S:    <iq type="result" id="RIdep-28" to="zxl1@user-20160421db/Spark"/>
 C:    TCP
 C:    <iq id="RIdep-29" type="get"><ping xmlns=‘urn:xmpp:ping‘ /></iq>
 S:    <iq type="result" id="RIdep-29" to="zxl1@user-20160421db/Spark"/>
 C:    TCP
 C:    <iq id="RIdep-30" type="get"><ping xmlns=‘urn:xmpp:ping‘ /></iq>
 S:    <iq type="result" id="RIdep-30" to="zxl1@user-20160421db/Spark"/>
 C:    TCP

场景三：zxl1添加zxl2,zxl2答应，zxl1答应,两者关系both
zxl2收到zxl1的请求，zxl2答应 订阅关系：zxl1:from zxl2:to
    同场景二的第1步
zxl1添加zxl2,zxl2答应，zxl1答应 ,订阅关系：both
 C:    <presence id="gGoyI-33" to="zxl2@user-20160421db" type="subscribed">
        <x xmlns="vcard-temp:x:update">
            <photo>d502a48d3e02f421269f9b1393c1f0cd825a8e4f</photo>
        </x>
        <x xmlns="jabber:x:avatar">
            <hash>d502a48d3e02f421269f9b1393c1f0cd825a8e4f</hash>
        </x>
    </presence>
 S:    <iq type="set" id="456-431" to="zxl1@user-20160421db/Spark">
        <query xmlns="jabber:iq:roster">
            <item jid="zxl2@user-20160421db" name="zxl2" subscription="both">
                <group>Friends</group>
            </item>
        </query>
    </iq>