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
* [ ] Add bootstrap template
* [ ] Write tests
* [ ] Remove items from drop down that are already selected
* [ ] Fix formatter/parser to work correctly on "multiple" mode (single property binding)

Demo [plunker](http://plnkr.co/edit/juqoNOt1z1Gb349XabQ2?p=preview)