CSS minification using YUI compressor

I wanted to use CSS compression so I've added yui-compressor and added a new `css_minify` argument. It works exactly like uglifer does. yui-compressor has been added as a development dependency only: it will still need to be manually added to the Gemfile of apps if you wish to use CSS compression.

I've also added tests to the existing suite.

I'm not sure if this is something this community is looking for, but I thought I'd share it back in case it is. :-)