Adds SCSS

This builds on @carlosrodriguez’s pull request. Uses the latest version of Node-sass to improve the install on OS X, Windows XP, 7 & 8, and then handles SCSS error messages.

Remaining issues aren’t really related to this:
* `@warn` and `@debug` is simply output to the console, which is more related to: https://github.com/sintaxi/terraform/issues/16
* Incompatibility issues with some libraries, which I’m testing: https://github.com/sintaxi/harp/issues/113
* SCSS files are still getting served, but that’s fixed here: https://github.com/sintaxi/harp/commit/66ca9613fde68ffc80a46724445de95b418c8716

The tests need to be updated for the Windows changes, but I didn’t know what to do there.