OAuth methods implemented.

Also changed SetupUI to PresentUI.
submodule update (Xamarin.Auth code moved to FacebookAuthenticator)
If session is active and FetchUserInfo is true, LogIn is the same as calling "me".
Added __MOBILE__ symbol, to use instead of __MonoCS__
submodule update (__MOBILE__ instead of __MonoCS__)
Implemented iOS-specific routines required for Xamarin.Auth presentation.
submodule update (FacebookSessionClient now partial)
New iOS-specific part of FacebookSessionClient

allows to pass UI elements where authentication screens will run.

NewFacebookAuthenticator is called from Xamarin-specific code so we can
assign UI stuff here.
Added a wrapper to the LoginAsync call, with default and iOS version.

iOS version of the wrapper sets up UI-specific parameters so
authentication screens know
where to show up.
Only define the default constructor if not in Xamarin build.

Platform-specific Xamarin builds must provide their own constructor
in their own partial.
Removed async; we are not calling await.
Fixes to small errors to make the project compile.
LogIn is an async Task, so we follow it with ContinueWith.

UpdateButtonCaption is not updated until the asynchronous LogIn Task is
finished.
Replaced calls to obsoleted iOS interfaces.
submodule update (added required castings to make this compile)