OAuth 2 Authorization Code Flow for Twitch.tv based on KatanaContrib.Security.Github

Made a few adjustments to support twitch oauth implementation:
- use url form encoded POST to get token
- update claims to match Twitch user object
- update default scope to match scope used by Twitch
- twitch requires the redirect_uri to match exactly, so make sure to set 'redirect Uri' to '<absolute url>/signin-twitch' in your twitch control panel