Add ability to pass custom libraries through to .use()

Added the ability to pass custom libraries, such as [autoprefixer-stylus](https://www.npmjs.org/package/autoprefixer-stylus) through to node-stylus-renderer, to utilise Stylus' [use](http://learnboost.github.io/stylus/docs/js.html#usefn) functionality without writing a custom compile function.

Changes:
* Added the ability to pass a single function, or array of functions to be `.use`d
* Removed Nib dependency from node-stylus-renderer
* Updated Documentation

To discuss:
* Removing Nib as a default means this PR will potentially break backwards compatibility, as Nib will now need to be passed through the new `.use` functionality if required.
* Writing tests for this new functionality
* Variable names (potential overuse of use/uses - couldn't think of anything better that was not overly long.)