Switch to Highlight.js

As discussed in #134.

I chose to switch to [highlight.js](http://highlightjs.org/) because I dislike the python dependency and how slow the invocation of child processes is. To render groc's own documentation, groc with pygments takes 9.5s on my machine, whereas groc with highlight only needs 3s (medium of 5 tests).

This pull request also includes my crude attempt at converting the original stylesheet to a highlight-compatible format (in SCSS) and adds the `highlightJS` key to some language definitions where the name highlight uses differs from pygment's.

<del>In general, I'm only changing the `highlightCode` method in `utils.coffee`. If you want to keep pygments available, I suggest you change `highlightCode` to call specific methods depending on an option, e.g. `highlightCodeHighlightJS` and `highlightCodePygments`.</del>

<ins>I've replaced `highlightCode` with `highlightCodeUsingHighlighJS` and `highlightCodeUsingPygments`. You can set your highlighter preference using the 'highlighter' CLI option or `.groc.json` property.</ins>

Stay tuned for my next PR to switch to marked.js.