brewkit.rb changes ENV destructively, so lets not do that everytime a formula
is required. Now it's possible for other tools to require a formula
description without worrying about side-effects.
I removed the rename and mv functions as when I wrote the tests I realised the function implied the pathname object would be updated to reflect the moved or renamed file. However that cannot be done. Also frankly I think writing it out in full makes clearer code.
