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!
instead of commenting and uncommenting the theme. We should find a way to set the value of the variables at runtime depending on the theme chosen by the user.

We could maybe create one class by theme. The user choose the theme by using a configuration property in his module configuration file. Then this class will read this configuration property and initialize its static fields by instancing the right theme class.


An other way should be to create one gss file by theme that will define the constants (no need to use a Java class) and then specify the theme you want in the `@source` annotation.
exemple:
defaultTheme.gss
```CSS 
@def C_PRIMARY = #fff;
@def C_PRIMARY_STEP1 #f4f4f4;
@def C_PRIMARY_STEP2 #eee; 
...
```

darkTheme.gss
```CSS 
@def C_PRIMARY = #000;
@def C_PRIMARY_STEP1 #0b0b0b;
@def C_PRIMARY_STEP2 #222; 
...
```

candylandTheme.gss
```CSS 
@def C_PRIMARY = #db2ddd;
@def C_PRIMARY_STEP1 #d52bd7;
@def C_PRIMARY_STEP2 #cb29cd; 
...
```
And people can define which theme to use in her own ClientBundle interface
```JAVA
@Source({"candylandTheme.gss", "mixins.gss", "icons/icons.gss", "chozen.gss"})
ChozenCss css();
```

I'm more in favor of the second option. 

@christiangoudreau