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.
Moved Forumla and Cellar/homebrew into Library.
This way the homebrew core files are more sensibly placed, Cellar is
more internally consistent and only generated. And Homebrew is ready for
use straight out of the tarball.
