Build script should create NuGet packages after a successful build

With these changes, build.cmd will automatically create NuGet packages for ScriptCs.Contracts, ScriptCs.Core, and ScriptCs.Engine.Roslyn if the solution builds and all unit tests pass. 

The .nupkg files will be placed in `artifacts\[Debug|Release]\` depending on the build configuration. The default build configuration is **Debug**. To create a release build, run `build.cmd /p:Configuration=Release`.

The package versions can be controlled by editing `build\ScriptCs.Version.props`.

#46