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.
One last remark: you should avoid using `Task.Result` and `Task.Wait` as much as possible as they always encapsulate the inner exception in an `AggregateException` and replace the message by a generic one (`One or more errors occurred`), which makes debugging harder. Even if the synchronous version shouldn't be used that often, you should strongly consider using `Task.GetAwaiter().GetResult()` instead.