on(eventType, callback) {
    this.client.prependListener(eventType, callback);
  }
Add a listner to the client object. This is rarely necessary and can be
the source of errors. It is the client's responsibility to remove the
listeners when no longer required. Failure to do so can result in memory
leaks.

@param {string} eventType - one of the supported event types
@param {function} callback - function called when event triggers