Autoprefixer (fixes #12452)

This is a work in progress! I'm posting it here to solicit feedback.

Currently it's a couple of separate commits to more easily see the different changes, but I can do a rebase if needed when it's done.

These commits add autoprefixer to the build tasks, removes the redundant manually prefixed styles from the less and docs files and does a grunt build to show the final output.

I have it setup to support the latest 2 versions of the major browsers, Internet Explorer 8 & 9, Android 2.3 (based on the comments on #12640) and Android 4 (which autoprefixer doesn't consider a major browser)

I had to split up the less Grunt task, as we want to do the prefixing after compiling the less files, but before minifying them. It also reprocesses the map files.

Most of the changes in the final css look okay to me, but I'm not entirely sure about the stuff it does to background-image.

What do you guys think?