Switched to Github API for avatars

Right now, we use the twitter API to get the avatars in the footer. This method seems to be broken with the twitter API 1.1. 

First, I was planning to [use gravatar](https://github.com/h5bp/html5please/commit/011de6605c342509a59cd120df4d51534bbd6074) for this, but some guys don't have an account on gravatar.

I think it's probably the best idea to use the github API for this, just like h5bp.github.io does.