Using SASS for CSS

After a [previous PR](https://github.com/harvesthq/chosen/pull/1103) about using LESS for CSS @pfiller asked me if I was interested in doing something alike in SASS. 

I've put down a list of things that (probably) need to be done:

- [ ] Replace manual vendor prefixes with mixins
- [ ] Split functional styling and visual styling
- [ ] Make Chosen somewhat customizable using variables
- [ ] Replace Compass with standalone mixins

If anyone has something to add to this list I'll be happy to hear so.

**NB** I'm currently using Compass for mixins, but as Patrick mentioned in his comment on #1103:
>  we won't be able to easily use Compass

So maybe later the needed mixins can be included in the SCSS itself, without the need of Compass.