initial revision of the GssResourceGenerator

This pull request contains the first draft of the GssResource and its generator.

What is done:
- should compile any gss file : all features offered by google closure stylesheet can be used (except conditionnal nodes because all conditional variables will be considered to be false)
- you can access the style class of a gss file by defining methods in the associated GssResource
- you can access the variable defined in the gss file (@def) by defining methods in the associated GssResource
- style classes names obfuscation 
- automatic RTL flipping
- implements SupportsGeneratorResultCaching

What remains to be done (first) : 
- add unit tests
- add missing features of CssResource
- find a way to use permutations axis with the conditional node
...


Please note that this is a work in progress and the main purpose of this first draft is to clear the ground