code: function (data, callback) {
      if (typeof callback !== 'function') {
          this.debug("Registering dummy callback, because a response message is expected");
      } else {
          this.debug("Registering callback");
      }
      this.waitingCallbacks.push(callback);
      send.call(this, data);
  }
docs: Send a message and register a callback
  @param  {Object}   data     Message to be sent
  @param  {Function} callback Callback function to be registered