chore(build): Adding Sass to Grunt pipeline

Adding Sass and Autoprefixer into the mix. This also means we need a dist/ directory, so things are about to get "weird".

Took a rough stab at documenting the available Grunt tasks, although some are subject to change once we get minification and other things involved.
@6a68 Basically what @johngruen says, but yes, it is very configurable if you only want to target specific browsers and needlessly screw over other people's preferred browsers for no reason.

https://github.com/postcss/autoprefixer#browsers

> _By default, Autoprefixer uses `> 1%, last 2 versions, Firefox ESR, Opera 12.1`_
> _Latest Firefox ESR is a 31 version._