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.
@nchapman No luck on that normalize lib (has a big Compass dependency, apparently). But I did manage to get https://github.com/hail2u/normalize.scss working in < 2m using this:

```css
@import '../bower_components/modularized-normalize-scss/normalize';
```

But AFAICT, the only thing that buys us is one less CSS request (and may cause some funny business when doing autoprefixer since it'd be trying to autoprefix normalize.css as well).
We can always revisit in a future PR if somebody thinks it's a useful experiment.