removeEventListener(eventName, listener) {
    var listenerArray = this._listeners[eventName];
    if (listenerArray !== undefined) {
      var index = listenerArray.indexOf(listener);
      if (index !== -1) {
        listenerArray.splice(index, 1);
      }
    }
  }
Remove an event listener
@param {String} eventName Name of the event to remove
@param {Function} listener Callback for the specified event