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 (maybe dropping support for this theme? https://github.com/angular-ui/ui-select/issues/175)
* [x] Add bootstrap template
* [x] Change transinject for uisTranscludeAppend from https://github.com/angular-ui/ui-select/pull/151
* [x] Improve margins between buttons at bootstrap theme
* [x] 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
* [x] 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
* [x] Refactor/Clean code 
* [x] Add demo
* [x] Avoid $setViewValue third parameter as explained on this [comment](https://github.com/angular-ui/ui-select/pull/138#discussion_r16508504)
* [x] Fix: refresh function, [every new search removes items selected earlier](https://github.com/angular-ui/ui-select/pull/138#issuecomment-52737817).

Demo [plunker](http://plnkr.co/edit/juqoNOt1z1Gb349XabQ2?p=preview) (I'll be updating it with each new commit)