CSS Minification

CSS now processed through clean-css. 

No source map, but getting source maps might be more complicated than I'm feeling like tackling at the moment. With give a shot first though. 
I'm :+1: to merge if we make a few tweaks to this file.

This method would be more consistent with the other utils if it looked more like this:
``` coffeescript
module.exports = (inputTree, options = {}) ->
  return inputTree unless env == "production"
  cleanCss(inputTree, options)
```

So basically a few style points for consistency with the rest:

1. I'd name the import "cleanCss" so that it maps directly to its package name (I'm lopping off the redundant "broccoli" off everything as well, though
2. I'd make `options` a default arg and not pass it from `css.coffee` because it's apparently a leaking implementation detail (so far) from its perspective

Broadly, I'm a little wary of figuring out what the right layer of abstraction is for all the (myriad) env-specific stuff we're sprinkling in. I wonder whether or not this is something we should drop down into application config and allow it to be overridden in the `<user>/config/{development,test,production}.js` files instead of only looking at the process env and branching on that. But that's a point we can deal with down the road in a separate ticket to clean up.