Fix Selectize theme

Selectize theme was not working.

Beside the cleanup, here the 3 important modifications:
- [src/selectize/match.tpl.html](https://github.com/tkrotoff/ui-select/commit/6d15b423ce7347ce9e9f139533addc5a9f0d6636#diff-acdd9c91e8963467e87217a152aadb97L1)
- [src/selectize/select.tpl.html](https://github.com/tkrotoff/ui-select/commit/6d15b423ce7347ce9e9f139533addc5a9f0d6636#diff-16fa5d5e011fdcfd97c85dd970a1f8c9L1)
- [src/selectize/choices.tpl.html](https://github.com/tkrotoff/ui-select/commit/d0e45f9cee87f020937ac2d2f6dd7e6403d9fddf#diff-7f6b9eeed1e6df04d952f3bd89b97cbaL1)

I have updated demo/index.html so that it displays 2 ui-select: one with the default theme (Select2), the other with Selectize theme.

I have not updated `gh-pages`, ~~I think it is better to simply rely on Plunker/JSFiddle + README.md (at least for now).~~