Xamarin support

This PR is to add Xamarin support for FluentAssertions

### TODO
- [x] Xamarin.iOS - Tests runs and passes on both simulator and device
- [x] Xamarin.Android - Tests run and pass on device
- [x] NuGet Nuspec updates

### Notes

Xamarin does not support MSTest, so if NUNIT is defined, the tests will use Nunit attributes to run