Make avatar_url available through the user object

The gravatar_id attribute has been deprecated in favour of avatar_url:
https://developer.github.com/changes/2014-09-05-removing-gravatar-id/

Removing gravatar_id entirely would break clients, but avatar_url should
at least be available.
Merge pull request #40 from eugeneius/avatar_url

Make avatar_url available through the user object