Install homebrew more carefully

Installs homebrew the way their installer script would do it, avoiding
things like failures if the target directory already has contents. This
is particularly useful for setting `$installdir` to `/usr/local`.

See [homebrew/install](https://github.com/Homebrew/install) for details.
Binaries only work for a specific cellar
Don't do a shallow clone, it causes issues
Don't generate binary urls with newlines
Fix homebrew repo spec
Fix indentation
Merge pull request #72 from boxen/homebrew-approved-installation

Stepwise homebrew installation