Add choices grouping using 'group by' expression

As described in #48, I've done options grouping.
Here is [plunker](http://plnkr.co/edit/qvVKhG?p=preview) where you can see this changes in action.
When using the down/up keys to move selected value in "grouped mode", it isn't working correctly since the activeIndex is not in same order are if the items aren't grouped. Maybe saving current group could help, but some refactor will  be needed at https://github.com/angular-ui/ui-select/blob/master/src/select.js#L225