Log out from PubSub Node

To logout from single node, user must send presence stanza with type unavailable:

  <pubsub xmlns='tigase:pubsub:1' node='princely_musings'/>

Component will send events to all occupants as described:

<message from='pubsub.shakespeare.lit' to='francisco@denmark.lit' id='foo'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='princely_musings'>
        <presence xmlns='tigase:pubsub:1' node='princely_musings' jid='hag66@shakespeare.lit/pda' type='unavailable'/>

If component receives presence stanza with type unavailable without specified node, then component will log out user from all nodes he logged before and publish events.