added API to change chat subject

JitsiConference.js

     }
 };
 
+/**
+ * Set new subject for this conference. (available only for moderator)
+ * @param {string} subject new subject
+ */
+JitsiConference.prototype.setSubject = function (subject) {
+    if (this.room && this.isModerator()) {
+        this.room.setSubject(subject);
+    }
+};
+
 /**
  * Adds JitsiLocalTrack object to the conference.
  * @param track the JitsiLocalTrack object.
         }
     );
 
+    conference.room.addListener(XMPPEvents.SUBJECT_CHANGED, function (subject) {
+        conference.eventEmitter.emit(JitsiConferenceEvents.SUBJECT_CHANGED, subject);
+    });
+
     conference.room.addListener(XMPPEvents.MUC_JOINED, function () {
         conference.eventEmitter.emit(JitsiConferenceEvents.CONFERENCE_JOINED);
     });

JitsiConferenceEvents.js

      * A user has changed it display name
      */
     DISPLAY_NAME_CHANGED: "conference.displayNameChanged",
+    /**
+     * Indicates that subject of the conference has changed.
+     */
+    SUBJECT_CHANGED: "conference.subjectChanged",
     /**
      * A participant avatar has changed.
      */

doc/API.md

         - USER_LEFT - a participant left conference. (parameters - id(string), user(JitsiParticipant))
         - MESSAGE_RECEIVED - new text message received. (parameters - id(string), text(string), ts(number))
         - DISPLAY_NAME_CHANGED - user has changed his display name. (parameters - id(string), displayName(string))
+        - SUBJECT_CHANGED - notifies that subject of the conference has changed (parameters - subject(string))
         - LAST_N_ENDPOINTS_CHANGED - last n set was changed (parameters - array of ids of users)
         - IN_LAST_N_CHANGED - passes boolean property that shows whether the local user is included in last n set of any other user or not. (parameters - boolean)
         - CONFERENCE_JOINED - notifies the local user that he joined the conference successfully. (no parameters)
 
 28. isStartVideoMuted() - check if video is muted on join
 
-29. sendFeedback(overallFeedback, detailedFeedback) - Sends the given feedback through CallStats if enabled.
-    - overallFeedback an integer between 1 and 5 indicating the user feedback
-    - detailedFeedback detailed feedback from the user. Not yet used
+29. setSubject(subject) - change subject of the conference
+    - subject - string new subject
+
+    Note: available only for moderator
 
 JitsiTrack
 ======

modules/xmpp/ChatRoom.js

     var msg = $msg({to: this.roomjid, type: 'groupchat'});
     msg.c('subject', subject);
     this.connection.send(msg);
-    logger.log("topic changed to " + subject);
 };