Added App settings support. for working with hidden settings on Azure Websites.

https://oauthclaimsdemo.azurewebsites.net/ 
http://www.s-innovations.dk/Blog/2013/04/26/Deploying-social-oauth2-claims-aware-websites-to-Azure-using-Git

It register providers from app settings when the area is being registered. I added it under the same flag, OAuth2Client.AutoRegisterOAuthCallbackUrl. It properly should have its own flag for enabling this behaviour. Hope you can use some of it.