Sort CSS declarations

[Siilwyn]

I propose to add CSS declaration sorting. Sorted declarations would make the gzipped size smaller because there will be more similar strings.

To make this even better I'm also thinking of adding an order in the sorting module that sorts rules based on what kind of value they can contain. Putting declarations that only have a few possible values on top will result in more 'duplicate' declarations and thus even a better gzip compression.

Thoughts?