Check Sass style

* Use open source SCSS-Lint https://github.com/causes/scss-lint
* Based on https://github.com/thoughtbot/hound/pull/406
* Use SCSS config from Bourbon Neat
  https://github.com/thoughtbot/neat/blob/master/.scss-lint.yml
I really want to see more tests that test our default styles, this way we catch any breakage on updates, or configuration tweaks. Does this sound like work for another PR, or should we handle that here?