Basic WebAuthenticationBroker on WinRT.NET.Forms.

Known issues:
 - The back button is still missing.
 - This WebAuthenticationBroker does not work for callbacks which return a 404 web response.
   This is due to the WinForms WebBrowser behavior.
 - GetCurrentApplicationCallbackUri() does not return a "ms-app://" Uri. It uses a relative Uri instead (which is invalid under Microsoft's Windows Runtime).