Change appveyor to package artifacts and nuget files

[jmquigs]

@wallymathieu can you please check this (and my other PR)

[wallymathieu]

Well, it looks OK. Other projects are using FAKE in order to copy the files into a bin folder :

• build.fsx
• appveyor.yml

But since the project structure has not changed much over the years the above approach is probably quite fine. 😃