Added WpfInteropSample which shows how to render into a WPF D3DImage.

This pull request adds a sample project which shows how to use MonoGame to render graphics into a WPF D3DImage. 

This sample works only with the changes of following MonoGame pull request: https://github.com/mono/MonoGame/pull/1497
It also needs SharpDX >= v2.5.0. It does not work with SharpDX v2.4.2 because of a bug where DeviceEx constructor throws a NullReferenceException.