Cache static files by hash

Implements #2734.

Deployment note: `GRATIPAY_ASSET_URL` needs to be changed to `https://assets.gratipay.com/` on Heroku. I don't think we'll need to change anything on the CDN side, can you confirm that @whit537 ?
Looks like MaxCDN supports it:

![screen shot 2014-10-27 at 5 51 05 pm](https://cloud.githubusercontent.com/assets/134455/4800177/68c84c2a-5e23-11e4-8634-ea00eced6303.png)