Give timelines a name

The debug of "joined timeline [Object] to timeline [Object]" isn't terribly
helpful, so let's at least give them an identifier.

lib/models/event-timeline.js

@@ -42,6 +42,8 @@ function EventTimeline(roomId) {
 
     // this is used by client.js
     this._paginationRequests = {'b': null, 'f': null};
+
+    this._name = roomId + ":" + new Date().toISOString();
 }
 
 /**
@@ -290,6 +292,15 @@ EventTimeline.prototype.removeEvent = function(eventId) {
     return null;
 };
 
+/**
+ * Return a string to identify this timeline, for debugging
+ *
+ * @return {string} name for this timeline
+ */
+EventTimeline.prototype.toString = function() {
+    return this._name;
+};
+
 
 /**
  * The EventTimeline class