Use github for auth

[h-lame]

I think it might be useful to use github for auth instead of twitter. We had problems last year when it turned out we didn't have anyone's email address for getting in touch (see Tom's last comment on [#2]). If we use github instead we can get their email address.

Also, it's all about devs, innit? Github is where devs are. Should be easy to just add omniauth-github and remove the twitter integration.

[lazyatom]

I believe this is done as of c25f6f3