Serve assets through the asset pipeline

porada/middleman-autoprefixer#4

Autoprefixer post-processes only those stylesheets that are served through the [asset pipeline](http://middlemanapp.com/basics/asset-pipeline/) (introduced [in Rails](http://guides.rubyonrails.org/asset_pipeline.html)). I changed the asset hierarchy and used some built-in helpers in order to make (Middleman) Autoprefixer work for you.

I made one bonus optimization which is concatenating the served stylesheets. I’d suggest you also [minified](http://middlemanapp.com/advanced/configuration/) them.