Added support for passing JToken-derived objects as custom objects

This change would allow consumers to add custom items like so:

    var notif = new AppleNotification()
        .ForDeviceToken(message.Token)
        .WithAlert(message.Text)
        .WithCustomItem("foo", JObject.FromObject(new { One = 1, Two = 2 }));

Currently this would throw an exception. This would provide a workaround to the following issues:

https://github.com/Redth/PushSharp/issues/236
https://github.com/Redth/PushSharp/issues/248