code: function subscribe(event, callback) {
  	/*jshint validthis: true */
  	if (DEBUG_DISPATCHER) log.info('SUBSCRIBE: ' + event);
  	if (!event || !callback) {
  		return this;
  	}
  	if (this._callbacks[event]) {
  		this._callbacks[event].push(callback);
  	}
  	else {
  		this._callbacks[event] = [ callback ];
  	}
  
  	this._client.subscribe(event);
  
  	return this;
  }
docs: Subscribes to a particular event. Anytime this event is published to a dispatcher, on any node in the system, the callback will be
  executed shortly thereafter.
  @param event A string name for an event, such as "org_14301" to be notified whenever that org is updated.
  @param callback A callback to be executed when the event is published.
  @returns {*} The context, for fluent calls.