Use gravatar for author and committer avatars.

# Summary
* add minified `md5.js` from https://github.com/blueimp/JavaScript-MD5 ([MIT](https://github.com/blueimp/JavaScript-MD5#license))
* add `avatar_default_url` config
* add `gravatarImage` url helper
* bind gravatar urls in `BuildView`

# Preview
![screenshot-by-nimbus 32](https://cloud.githubusercontent.com/assets/193936/2923197/8bfb14a2-d70f-11e3-9e17-227f44bc4dde.png)