CSS to GSS, sprite images to webfont, color variables, mixins

A lot have been done : 

- Converting chozen.css to chozen.gss
- Adding every colors into Colors, making theming easier (there's a Dark theme and Candyland theme, just uncomment them to see in action)
- Converting the sprite image into a font, so we can change the elements colors and size with CSS (perfect for theming)
- Adding mixins
- Removing unused css classes
- Cleaning code here and there

Before merging, I still need to check for RTL settings and your wonderful comments!
Yep maybe we should not provide theme for chosen. But the problem with the current implementation is that the user won't be able to change the colors at all. The static fields of the class are final. So the user cannot change the values of these fields.