Added select2-loaded event trigger in loadMoreIfNeeded

The 'select2-loaded' event does not fire when subsequent items are loaded in the loadMoreIfNeeded function.  This uses the same convention as #1455:

    this.opts.element.trigger({ type: "select2-loaded", items: data });

However, I would also recommend that both be replaced with the second suggestion in #1455 to use the jQuery data field instead of a unique 'items' property:

    this.opts.element.trigger({ type: "select2-loaded"}, data);