This fixes #390.
@connors: don't merge this, just review and I'll merge myself after rebasing.
This requires Sass 3.3 so it's on hold for now.
On a side note, I tried this with #479 and it doesn't seem to work with "grunt-sass".
Use the `@at-root` directive to work around the issue.
Add csslint in the test target.
Re-enable clean-css's advanced optimizations.