Added SCSS with Compass as a CSS Preprocessor

I am finally done with this, almost forgot it! I went from LESS to SCSS due to it being more popular, projects like yeoman support it. Eric Meyer is one of the developers for Compass (http://compass-style.org). It is well tested, and it is what I will be using in the future. More about SCSS: http://sass-lang.com/

What I did:
- I rewrote the CSS as CSCC with Compass mixins
- Result indent is a variable.
- Border-radius is a variable.
- Updated the readme with a quick guide to SCSS

What can be done In the future:
- Automatic generation of sprites, both the normal one and retina one. (Compass)
- Better use of variables to quickly change colors.
- Simple CSS transitions via mixins?

Issue: https://github.com/ivaynberg/select2/issues/143

Any feedback? Just comment!