CSS optimisations & responsive image abstraction

- [new] `.u-responsive-ratio` helper to create 5:3 ratio placeholders
- [fix] Performance: prevent fromage normal images from upgrading/downgrading at all (since 220px is their original size we don't need to pass them to imager.js)

![ ](http://media2.giphy.com/media/cA9q5AUOWsnSg/200.gif)