Implement a full authentication handler

[Tratcher]

To enable mixed anonymous and windows auth the PlatformHandlerMiddleware must implement the IAuthenticationHandler contracts for Authenticate, Challenge, and Forbidden. Open questions:

• Should it be 'automatic', always processing the incoming identity and always responding to Challenges? This may need to be configurable.
• How can it know what auth is enabled in IIS? This may need to be configured too.
• When there is a WindowsIdentity, how can it tell what type of auth was used to put in the AuthScheme field? This will also be needed for converting challenges to forbidden.