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.
It seems like we might want to have a `build` task as part of this PR that runs most of these in the right order. We could then potentially document these as a hierarchy to make it clear what matters most.