feat(XMPPEvents): add CALL_ENDED event

The event is triggered when Jicofo sends 'session-terminate' and our
PeerConnection is closed.

JitsiConference.js

     // both camera and microphone.
     this.statistics.startCallStats(jingleSession, this.settings);
     this.statistics.startRemoteStats(jingleSession.peerconnection);
-}
+};
+
+/**
+ * Handles the call ended event.
+ * @param {JingleSessionPC} JingleSession the jingle session which has been
+ * terminated.
+ * @param {String} reasonCondition the Jingle reason condition.
+ * @param {String|null} reasonText human readable reason text which may provide
+ * more details about why the call has been terminated.
+ */
+JitsiConference.prototype.onCallEnded
+= function (JingleSession, reasonCondition, reasonText) {
+    logger.info("Call ended: " + reasonCondition + " - " + reasonText);
+};
+
 
 JitsiConference.prototype.updateDTMFSupport = function () {
     var somebodySupportsDTMF = false;

JitsiConferenceEventManager.js

     var conference = this.conference;
     conference.xmpp.addListener(
         XMPPEvents.CALL_INCOMING, conference.onIncomingCall.bind(conference));
+    conference.xmpp.addListener(
+        XMPPEvents.CALL_ENDED, conference.onCallEnded.bind(conference));
 
     conference.xmpp.addListener(XMPPEvents.START_MUTED_FROM_FOCUS,
         function (audioMuted, videoMuted) {

modules/xmpp/strophe.jingle.js

                         reasonText = $(iq).find('>jingle>reason>text').text();
                     }
                     this.terminate(sess.sid, reasonCondition, reasonText);
+                    eventEmitter.emit(XMPPEvents.CALL_ENDED,
+                        sess, reasonCondition, reasonText);
                     break;
                 case 'transport-replace':
                     var now = window.performance.now();

service/xmpp/XMPPEvents.js

     // Designates an event indicating that an offer (e.g. Jingle
     // session-initiate) was received.
     CALL_INCOMING: "xmpp.callincoming.jingle",
+    // Triggered when Jicofo kills our media session, this can happen while
+    // we're still in the MUC, when it decides to terminate the media session.
+    // For example when the session is idle for too long, because we're the only
+    // person in the conference room.
+    CALL_ENDED: "xmpp.callended.jingle",
     CHAT_ERROR_RECEIVED: "xmpp.chat_error_received",
     CONFERENCE_SETUP_FAILED: "xmpp.conference_setup_failed",
     // Designates an event indicating that the connection to the XMPP server