title: Strophe.muc plugin and backbone inproper callback binding
body: <p>After the first group message the strophe.muc plugin doesn't react on the following messages. 
  I get the first presence, message and roster from the room, but all subsequent messages and presence stanzas will not trigger the functions. 
  I think there is something wrong with the lines:</p>
  
  <pre><code>this.rooms[room].addHandler('message', msg_handler_cb);
  this.rooms[room].addHandler('presence', pres_handler_cb);
  this.rooms[room].addHandler('roster', roster_cb);
  </code></pre>
  
  <p>my code: </p>
  
  <pre><code>    join: function(){
            console.log("joining room");
            var self = this;
            connection.muc.join("room@conference.server", "john", self.onMessage,  self.onPresence, self.onRoster);
        },
  
      onMessage: function(message){            
            var self = this;       
           var body = $(message).text();
           var from = $(message).attr("from");
           console.log(body);
           console.log(from);
        },
        onPresence: function(presence){
            console.log("onPresence");
            console.log(presence);
        },
        onRoster: function(roster){
            console.log("onRoster");
            console.log(roster);
        },
  </code></pre>