Switched to libsass from ruby sass

`grunt-sass` has a very similar interface to `grunt-contrib-sass` so most of the options should be similar, just the compression stuff that needs changing.

Bare in mind that libsass (via node-sass, via grunt-sass) doesn't have feature parity with the latest ruby sass at the moment, so it won't have all the wizzbang features yet.

Fixes #1346