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!
This function is not a standard function of CSS and GSS should complain about it. 

You need to add the following line in chosen.gwt.xml:
```xml
 <extend-configuration-property name="CssResource.allowedFunctions"
                                 value="progid:DXImageTransform.Microsoft.gradient" />
```