Extract common OAuth2 middleware base class

[Tratcher]

Facebook, MicrosoftAccount, Google, all use OAuth2 and should derive from an OAuth package with shared base classes for middleware, notifications, notification contexts, etc..

Specific items of concern: Shared base class for *AutehnticatedContext with shared OAuth fields like AccessToken, RefreshToken, Id, etc.. See @blowdart for specific fields.

Stretch: Can this work for Twitter also? They use OAuth1 but many of the shared fields are the same.

FYI: There are also all of these 3rd party implementations that could be simplified:
https://github.com/owin-middleware/OwinOAuthProviders/tree/master/Owin.Security.Providers

[kevinchalet]

Here's the PR: #56 😄