Starting to simplify the OAuth2.0 classes

- Created a new abstract base OAuth 2.0 base class because a few of these providers are actually sharing the same code in various large chunks.
- FacebookProvider refactored to inherit from the new OAuth 2.0 base provider class.
MvcAuto website sample now shows lots of provider options.
Cleaning up the OAuth 2.0 providers.

- New AccessToken class.
- Refactored UserInformation class to have an AccessToken property.
- Refactored RetrieveAccessToken(..) to now return an AccessToken instance.
- OAuth 2.0 providers updated to work with the new RetrieveAccessToken signature.
- WindowsLive provider now inherits the BaseOAuth20Provider.