Adds NuGet package building to end of build and separates out Native NuGet project.

The Native libraries now ship in a separate NuGet package which the managed NuGet package depends on.  All package building is done as part of the build process and managed with MSBuild targets.
Makes Native NuGet package copy the native libraries to $(TargetDir) on build of the project the package is added to.
Don't re-copy unchanged files (they should basically never change except on clean).