Added missing Microsoft.Xna.Framework assembly reference to Cimbalino.Phone.Toolkit.MediaLibrary.nuspec
Added some new extension methods to IntExtensions
Removed the WriteableBitmap.Invalidate() call from WriteableBitmapExtensions.SavePng()

Invalidate() should still be called before using the SavePng() extension method (same behavior as the platform SaveJpeg() extension method) and by removing it we can now call the method from a background thread