Add autoprefixer for CSS, swap sqwish for better clean-css

This might need a little bit more thorough testing than I've given it, but it seems to work for me.

Two things here:
 - Add [autoprefixer](//github.com/ai/autoprefixer) to CSS compilation for automatic vendor prefixes and expansion of things like keyframes
 - Swap sqwish for [clean-css](https://github.com/GoalSmashers/clean-css) because it produces smaller outputs and is a bit more intelligent than sqwish.