Review: Add choices grouping

**WIP**

This PR is based on https://github.com/angular-ui/ui-select/pull/49 but with some changes based on the comment I left there.

Basically navigation with up/down key is fixed in "grouped mode"

TODO:
* [ ] Some tests are failing
* [ ] Make a demo with iterator *function*
* [ ] Choices template needs to show correctly only simple/grouped block 
* [ ] Check correct format of all themes 
* [ ] Update README with demo of this feature

Demo [plunker](http://plnkr.co/edit/kmRoNPwhxZYkjjHfFxhq?p=preview)

cc: @just-boris