In makeTree, if absolute use the given root

Fixes Windows, where calling makeDirectory on a drive (e.g. "C:\")
fails with a permision error. This will cause the drive to be skipped
on Windows. "" will be skipped on *nix, which when joined with the
next part turns into "/", the same behaviour as before.
Merge pull request #25 from Stuk/makeTreeRoot

In makeTree, if absolute use the given root