Twitch.tv provider

I added this very basic oauth2 provider for twitch.tv. 
Docs for the twitch.tv api can be found here: https://github.com/justintv/Twitch-API/blob/master/authentication.md

I might improve this provider in the future (like using `/user/:username` instead of `/user` to get the profile if user_read isn't set). But it should work good as it is now.