function callbacksFor(object) {
  var callbacks = object._promiseCallbacks;

  if (!callbacks) {
    callbacks = object._promiseCallbacks = {};
  }

  return callbacks;
}
Registers a callback to be executed when `eventName` is triggered
     ```javascript
    object.on('event', function(eventInfo){
      // handle the event
    });
     object.trigger('event');
    ```
     @method on
    @for RSVP.EventTarget
    @private
    @param {String} eventName name of the event to listen for
    @param {Function} callback function to be called when the event is triggered.