Added support for .styl stylus assets

This pull request adds stylus support to sails and introduces a minor refactoring of the css assets code which should make it easier to add other css languages (although they are still hardcoded into assets/index.js.. that should probably change at some point).


I realize there is a discussion going on in https://github.com/balderdashy/sails/issues/240 about how best to integrate with yeoman. As a sails newb I did not want to complicate things by trying to learn two frameworks at once (and yeoman does not generate simple code) so I built this to keep things simple and sails-centric.


Cheers,

Rob