fix: data not passed in select2-loaded event

When triggering `select2-loaded` event items are passed via `data` property that is standart $.Event field and can not be modified in trigger. So that data remains `undefined`:
````js
this.opts.element.trigger({ type: "select2-loaded", data: data });
````
jsFiddle: http://jsfiddle.net/Agdpu/

I've renamed it to `items` and everything works:
````js
this.opts.element.trigger({ type: "select2-loaded", items: data });
````

But basically, I would suggest to think about passing parameters to all events via second argument of `trigger`:
````js
this.opts.element.trigger({ type: "select2-loaded"}, data);
````
It's more recommended way by jquery and avoids such bugs with naming.