CheckBox with WP8 renderer, based on Xamarin.Forms 1.1.0.6201

The control is based on the latest Xamarin release which introduced some breaking changes and is not backwards compatible. I didn't update the rest of the project, just the two files I updated. See http://forums.xamarin.com/discussion/18361/xamarin-forms-1-1-0-6201-released for more info.