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!
If it's provided, users will always need to import gssresource explicitly. If it stays compile, it will be transitive. Since gwtchosen is to be provided in users' projects, gssresource will become provided as well. I think it saves a lot of trouble to keep it compile.