Optional Compass/Sass support

I prefer Sass to Less, and use compass to preprocess everything and minify it; so I modified the Gruntfile so it can do so with minimal effort.

The defaul is still less, you just need a change in settings to choose one or the other (Althourh I reckon that once you chose one path or another, you'll just remove the other css preprocessor).

Also added a parameter in the git clone command in "build-plugin", so it brings as little history as possible before rm-fing ".git".

Regards!