Support for multi-selection

**WIP:**

Based on work done at https://github.com/angular-ui/ui-select/pull/78 by @komakino. I manually rebased and did some minor changes.

TODO:
* [ ] Add selectize template
* [x] Add bootstrap template
* [ ] Change transinject for uisTranscludeAppend from https://github.com/angular-ui/ui-select/pull/151
* [ ] Improve margins between buttons at bootstrap theme
* [ ] Write tests
* [x] Remove items from drop down that are already selected
* [x] Check formatter/parser (single property binding) to work correctly on "multiple" mode
* [ ] Check choices with groupby to work correctly on "multiple" mode
* [x] Fix: multiple choices highlighted at same time
* [x] Fix: dropdown doesn't appear when pressing key down (just the word "true") 
* [x] Fix: when pressing a key on the input, the whole selected array is erased
* [x] Prevent already selected items to show when searching
* [ ] Refactor/Clean code 

Demo [plunker](http://plnkr.co/edit/juqoNOt1z1Gb349XabQ2?p=preview) (I'll be updating it with each new commit)
As mentioned in https://github.com/angular/angular.js/issues/8481 --- please do not use the third parameter, this is most likely going to be removed soon and won't do what you expect, and it only really does anything at all in a few (beta) releases of angular.