Remove vendor prefix mixins

This fixes #54 and #59.
Autoprefixer is now required. I've added the Grunt config as per the official Bootstrap config.

This fixes most of the inconsistencies between the Stylus & Less versions with vendor prefixes, but there is still a few extra `filter: alpha(opacity=...)` rules in the output. Not that it matters much.

I also fixed up a couple things I missed with popovers & tooltips.