Accurate crossword thumbnails (the animated GIF will explain all)

![crosswordz](https://cloud.githubusercontent.com/assets/1001642/4835113/0aa75b6c-5fb4-11e4-97aa-a2596f3d9731.gif)

@garynewby gave me the idea.
We are not putting images on www.theguardian.com...

One of the big R2 mistakes was serving everything off of there. Once a url is on www.theguardian.com we cannot remove it.

I don't mind this code being here, but you need to setup something appropriate on `i.guim.co.uk` to route to it.