fix the bug initSelection not being called when multiple=true

the problem is ui-select2 init select2 plugin before it init the element val, and select2 do not call initSelection if the val of the element is empty.
so i change the line order to first init the element val and then start select2

plunker that contain both my fix and the original directive, comment out one of them and config the the parameters

http://plnkr.co/edit/T2hpQlUoL2qJAXIsgt8T