code: function(callback, contextOverride){
      this._callbacks.push({cb: callback, ctx: contextOverride});
  
      this._deferred.done(function(context, options){
        if (contextOverride){ context = contextOverride; }
        callback.call(context, options);
      });
    }
docs: Add a callback to be executed. Callbacks added here are guaranteed to execute, even if they are added after the `run` method is called.