Use Gravatars. Fixes #127

Uses [CryptoJS](https://code.google.com/p/crypto-js/) to generate a md5 hash of the author's e-mail for the gravatar img src.

Let me know if there is a better alternative to [CryptoJS](https://code.google.com/p/crypto-js/), that you would like to use instead, and I can make the appropriate changes.