added animation-play-state prefixes (fixed hover-stop on chrome)

Using latest version, I noticed shake-constant and hover-stop didn't work as expected on Chrome, since an animation-play-state prefix is required for webkit.

I don't know much (at all!) about SASS, so you'll most probably want to rewrite this, but it works for me. Also note the output css and .min.css were generated clumsily but I included them both for completenes sake.