[windows] better implementation for NuGet files (#25)
[windows] better implementation for NuGet files (#25)

My previous implementation of Windows/NuGet had some issues:
- The `<Copy />` tasks can cause double writes (only a perf issue)
- If using `libZipSharp.csproj` as a `<ProjectReference />`, the NuGet
dlls are not copied to the other project's output directory
- I was using the `<Touch />` task to create an empty file

I now have alot more MSBuild knowledge, changes include:
- Just make `$(DllConfigOutputFile)` conditional, we can remove the
`_UpdateDllConfigNotUnix` target
- Use `<Content />` and `CopyToOutputDirectory` to fix
`<ProjectReference />` in other projects
grendello/LibZipSharp