added support to configure sass compiler, sassc is way fast

I am working on a new theme using [Foundation 5](http://foundation.zurb.com/) using the sass sources. Compiling with the Ruby implementation of sass is sort of slow, and I wanted to be able to use [sasc](https://github.com/hcatlin/sassc) which is implemented in C++ and is way faster.

This pull request adds a configuration option SASS_COMPILER which defaults to 'sass' but allows you to configure which compiler you want to use.