Generate CSS from SASS

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:

- [x] Replace manual vendor prefixes with mixins
- [x] <del>Split functional styling and visual styling</del>
- [x] Use shorthand color codes
- [x] Use unprefixed versions of properties (`border-radius`, `box-shadow`, `text-overflow`, `background-clip`)
- [x] Remove IE specific `filter:`

Bonus features:
- [ ] Make Chosen customizable using variables

If anyone has something to add to this list I'll be happy to hear so. As a follow up on #1306 I would like to hear your opinion also @lukeapage.

<del>**NB** I'm currently using Compass for mixins, but as Patrick mentioned in his comment on #1103:</del>
>  <del>we won't be able to easily use Compass</del>

<del>So maybe later the needed mixins can be included in the SCSS itself, without the need of Compass. </del>