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.
I only use:
- `$ grunt dist` &mdash; Cleans and rebuilds /dist/ dir.
- `$ grunt css` &mdash; Sass->CSS and autoprefixes into the /dist/ dir.
- `$ grunt lint` &mdash; You can run manually to run full suite of linters, or it should get run automatically as part of the precommit-hook script.

But yeah, @nchapman is right, maybe I should create a `build` task which runs `dist` and `sass` for now.

As for the other tasks, I think we'd only run `changelog` and `contributors` when deploying a new version/tag to GitHub and `validate-shrinkwrap` gets run on Travis or whenever you modify the npm-shrinkwrap.json file via `npm run shrinkwrap`.

And apparently `watch` isn't wired up yet, so I'll file a new tracking bug for that. I think the plan was that it'd watch the Sass files and recompile those if they changed. Similarly I guess changing a JS or JSON file should rerun the linters.