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.

Git Formula doesn't build anything from a formula, so allow this usage.
It feels dirty though :(

Basically Formulae are now classes that get dynamically loaded by the
brew script. This gives me more flexability.