Created a helper function `tarFiles`

[lehins]

As described in haddock tarFiles is a very helpful function for adding files and folders into the tarball. Most importantly it can:

• recurse into directories down to a specific level
• place files into subfolders, by prefixing all names and adding missing prefix folder structure
• strip prefix from files that are being tarred, which is sort of like cding into the prefix folder, but not exactly
• figure out from files being tarred if any missing directories need to be added in order for the tarball structure to stay valid.