code: function(eventName, callback) {
          var allCallbacks = lib$rsvp$events$$callbacksFor(this), callbacks;
  
          callbacks = allCallbacks[eventName];
  
          if (!callbacks) {
            callbacks = allCallbacks[eventName] = [];
          }
  
          if (lib$rsvp$events$$indexOf(callbacks, callback) === -1) {
            callbacks.push(callback);
          }
        }
docs: 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.