Rewrite taking advantage of Stylus

I understand if you reject this, since it _is_ a pretty big change, but since I needed to write this for [nib](https://github.com/visionmedia/nib) anyway, I'm submitting this PR in case you are interested.

What I did:
 - remove all the grunt stuff, since stylus can do concatenation on it's own, and users usually have their own build systems
 - reorganize the readme, moving the license and contrib sections into their own files
 - rename the css files to stylus so they can be included directly into other stylus files
 - add examples of flexible including with the Stylus-implemented glob-star

This is migrated from https://github.com/daneden/animate.css/pull/135, since I rebased the whole thing after the rewrite you mentioned.