Adds a new DownloadStrategy that checks files out from Subversion to the
cache, then exports over to the temp folder for the build.
To use checkout with the svn:// protocol or reimplement download_strategy and
return SubversionDownloadStrategy.
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.
