docs: 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;
  }