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.
Consider removing this setting as:
* It's not part of the OAuth2 specs and definitely not mandatory.
* It might make one feel that this handler is able to automatically/magically retrieve the profile, which is not the case if you don't provide the adequate notification delegate.