Sass support and Compass extension.

This PR makes small changes to the Less that allows it to be converted to Sass. Some Less features do not map neatly to Sass: in this case, it's the interpolation of & in the selector string.

A Gruntfile is added to describe this libraries tasks (including the existing less compilation task).

grunt less:build - builds new CSS from the Less source
grunt sassify - Converts existing (currently only Baseline 2) less files to Sass.
grunt test - builds Less and Sass to test/output and checks saved diffs to see if they've resulted in expected changes

The Sass files are made available as a Compass extension.

Because this adds new functionality without breaking backward compatibility, a suggested minor version bump commit is included.