Octo.exe pack to include empty folders

[KennethBates]

Currently when using the Octo.exe pack command (either into Zip or NuGet), empty folders aren't included into the package contents. We should allow empty folders to be packed with everything else.

NuGet has a flag to include empty folders (ExcludeEmptyDirectories) which we don't use, and the file enumerator for the Zip code has an option to include empty directories.

Probably want to include it into the build server plugins too.

[Trisped]

It should be noted that this feature should use a new flag to indicate if empty folders should be included.