Implement Stylus in place of Sass

This is certainly up for some discussion. I think we ought to swap out Sass in favor of Stylus.

## Reasons for using Stylus of (lib)sass

- We're currently using [libsass](https://github.com/sass/libsass/), the C version of Sass. It's a bit behind in comparison to native Sass. And there's no sense in using native Sass because it adds an entirely new language to the process, Ruby. Stylus is based completely on Node while we use a Node port of libsass.
- Because this version of Sass is a Node iteration of a C iteration of a Ruby-based compiler, new changes have to work their way through all of these repos for us to use new features. Stylus, being JavaScript-based already, doesn't need to work through all of these plugins to use new features.
- Stylus is more active. There's over [100 collaborators](https://github.com/LearnBoost/stylus/graphs/contributors) on the Stylus GitHub project while libsass has about [half that amount](https://github.com/sass/libsass/graphs/contributors).

## Things to note

- The Stylus documentation does not use `;`, `:`, or `{ }` in their examples. I feel that's excessive. I propose we still continue to use these characters so developers can easily transition from CSS in Stylus with little issue.
- All the common parts of Sass exist within Stylus like [variables](http://learnboost.github.io/stylus/docs/variables.html), [mixins](http://learnboost.github.io/stylus/docs/mixins.html), [extend](http://learnboost.github.io/stylus/docs/extend.html) and [more](http://learnboost.github.io/stylus/).