Added support for autoprefixer

Before I lose track of it, I pulled out my autoprefixer setup for #62.

Currently, this only works for files processed by compass, because it only runs on files in `.tmp`. In order to make sure files aren't processed twice by autoprefxier, we would have to restructure the tasks a little to make it work for plain CSS files. OTOH, I would argue if you use plain CSS, you might not want any magic at all.

But let's discuss those details post-1.0. :)