Use '//' style comments, nesting and `@extend`

There's more work that can be done here, but it's certainly a start.
This commit shouldn't break any of the existing styling.
Translate scss files to sass syntax

Sass syntax is a whitespace and indentation based. I think it looks a
little prettier.
Merge pull request #39 from routemaster/features/more-sassy

Clean up the CSS by using more SASS features