Image versioning

@thomas-huston-zocdoc

Adds option image versioning to the final minified css files so that we don't get bitten by browser caching on our sprites.

Images that it can verify on disk are rewritten from:

`images/an-image.png` 

to 

`combined-directory/version__THEHASHOFTHEIMAGE__/images/an-image.png`

Server side we can remove `combined-directory/version__THEHASHOFTHEIMAGE__/`, do a context rewrite, and route the request to the correct file.