Add ability for mock to accept working directory

and add test
Add failing test for makeTree

it should make an absolute tree from a subdirectory
Fix makeTree on *nix

My understanding of join() was wrong, it is not the inverse of
split(), as it ignores any parts which are an empty string.

This fix will correctly use the root instead of an empty string.
Merge pull request #26 from Stuk/maketree-fix

Maketree fix