Cleanup extension, remove Compass/Sprockets dep.

Hi there. Love the extension, but I wanted to get it prepared for the next major release of Middleman coming later in the year. We're making Sprockets and Compass optional, so extensions should be able to function without them. This PR makes that work and adds some additional options from the `minify_css` extension.

Here's what changed. Thanks again!

* Allow auto-prefixer to work directly on CSS content, rather than relying on Compass or Sprockets.
* Support prefixing CSS content inline inside HTML. With tests.
* Remove `middleman_extension.rb` which is only used for extensions with CLI templates.