Fix and export Setup

Setup function is legacy function used to setup files on TestDrive.
Discovered that this does not work properly running it against slightly
modified Chocolatey code base. They assume Pester 2.0.2 in their test
base so some fixes are needed to get most of their test base work. They
also use some of the Pester functions that were meant to be internal.
Like cleanup. This made me realize that the changes we (ok mostly you
:)) do might be bigger that expected and all the changes should be
properly documented/communicated to the users.

Minor documentation changes.