[NuGet] Add support for Xamarin target frameworks.

Xamarin target frameworks:

    Xamarin.iOS
    Xamarin.Mac
    Xamarin.PlayStation3
    Xamarin.PlayStation4
    Xamarin.PlayStationVita
    Xamarin.Xbox360
    Xamarin.XboxOne

Source code changes:
    Commit: https://github.com/mrward/nuget/commit/0225161b94a87e720d2d95d43a3b08156c9c36cf