code: function( id, callback ){
  		var ID = id;
  
  		// If the callback is the first parameter
  		if( typeof id == 'function' ){
  			ID = 'ID_' + this._ID;
  			callback = id;
  		}
  
  		this._callbacks[ID] = callback;
  		this._ID++;
  
  		return ID;
  	}
docs: Register a callback that will be called when an action is dispatched.
  
  @param  {String | Function}   id  If a string is passed, it will be the id of the callback.
  If a function is passed, it will be used as callback, and id is generated
  automatically.
  @param  {Function} callback If an id is passed as a first argument, this will be the callback.
  @return {String}            The id of the callback to be used with the waitFor method.