Allow Github as OAuth authentication

[xan2622]

I know it's already possible to log in with Indieroyale, Dailyroyale, Facebook, Twitter, Google or Steam accounts but it's not currently possible to do it with Github.

OpenRA (the project I like and I support) already has a Desura installer:
http://www.desura.com/games/openra

OpenRA fans (players) often post feature requests and bug reports on Github (the OpenRA source code is also on Github: https://github.com/OpenRA/OpenRA) and they may be interested in using their Github account with Desura too.

Please, help us make our Desura installer more popular by adding this option.

Thank you.