Adds support for use as Stylus plugin

This pull request is based mainly on comments and additions suggested in #72 .

The highlights are:

* Stylus plugin support as added by @eafzali
* Updates the directory structure to add some sort of namespacing to avoid import conflicts as mentioned by @Aaike
*  Instructions on downloading via npm and basic usage within the read me
* Updated package information specifically keywords, github url and description

I've run basic tests scenarios and they seem to pass. Obviously this pull request has been made before there was a consensus on renaming the directories but I needed this functionality now so I've implemented it in a fork. It would be preferable if this was in the main repo.

Feedback welcome.