Switch to an AST based parser and builder

Starts using [visionmedia/css-parse](https://github.com/visionmedia/css-parse) and [visionmedia/css-stringify](https://github.com/visionmedia/css-stringify) for
parsing the source and building the results. This avoids the messy and
hard to modify RegExes used to parse the source and makes it easier to
address issues like #19 or #18, adds the ability to add source maps and
fixes #8.