Add support for Xamarin target frameworks.

Xamarin target frameworks:

    Xamarin.iOS
    Xamarin.Mac
    Xamarin.PlayStation3
    Xamarin.PlayStation4
    Xamarin.PlayStationVita
    Xamarin.Xbox360
    Xamarin.XboxOne
Merge branch '2.8.1-monodevelop-xamarin-frameworks' into 2.8.1-monodevelop-unified-api-components