Adding support for Xamarin.iOS, Xamarin.Android

This adds client libraries for Xamarin.iOS and Xamarin.Android. The changes here are pretty minimal, and mostly just consist of some `#ifs` in a few of the same places Windows Phone needed them. The other modifications were to work around cases where Json.NET's `JToken.Value<T>()` call would cause the Xamarin.iOS client to crash due to JIT.

For now I'm only including the libraries themselves, and leaving them out of the main solution files to limit the disruption.

The Json.NET libraries for Xamarin.iOS and Xamarin.Android were taken from the Xamarin Component Store, and are under the MIT license: https://components.xamarin.com/license/json.net