OAuth base class

1. This abstracts common protocol operations into a series of base classes (see Facebook, Google, Microsoft), reducing duplicate code and making it easier to implement other providers.
2. The base class can be used directly to access other auth providers. See the GitHub sample in Startup.cs. Because resolving user information is not part of the OAuth protocol, that step must be left to the developer (see OnGetUserInformationAsync notification). If they don't, by default we will put the access token into the ClaimsIdentity for the application to utilize later.
Yep. Making it non-settable wouldn't change anything in this particular case.

This has always been a good practice to make collections properties read-only (http://msdn.microsoft.com/en-us/library/ms182327.aspx), but if you wanna keep it settable, I'm fine.