Add RoomState.getLastModifiedTime() and JSDoc

lib/models/user.js

@@ -78,6 +78,12 @@ User.prototype._updateModifiedTime = function() {
     this._modified = Date.now();
 };
 
+/**
+ * Get the timestamp when this User was last updated. This timestamp is
+ * updated when this User receives a new Presence event which has updated a
+ * property on this object. It is updated <i>before</i> firing events.
+ * @return {number} The timestamp
+ */
 User.prototype.getLastModifiedTime = function() {
     return this._modified;
 };