Proof of concept for creating aspnet50 and aspnetcore50 based assemblies to include in the generated nupkeg

This is just a (more or less) quick writeup to create the necessary infrastructure that creates the aspnet50 and aspnetcore50 assemblies for the nupkeg. This needs a KRuntime / VS2015 (https://github.com/aspnet/Home) to build.

I don't expect this to be merged. It just shows that providing those new targets through the nupkeg should not be too much of a hassle.