Added project Starcounter.ErrorCodes that will define what goes into the NuGet package with the same name, and what properties to use when packaging it. Also included a dummy .nuspec to use.
Added Starcounter.ErrorCodes to Starbits solution.
Enabled the build script to build a nuget package out of the Starcounter.Errors project (building currently fails though, since we don't define any output).