Moved Xamarin.Auth invocations inside FacebookAuthenticator.

This part except for the UI stuff is theoretically complete.
Use __MOBILE__ instead of __MonoCS__ to conditionally compile
Xamarin-specific code.
Made FacebookSessionClient partial, for cross-platform implementation.

Xamarin-specific LogIn part now calls a platform-specific
authenticator instantiator so UI-specific coding can be incorporated.
Added explicit casts for non-dynamic json data extraction.

Implicit casting doesn't take place.