Install native Homebrew packages

Buzzy can now find and install native packages from Homebrew.  We parse
the results of `brew info [package]` to determine which versions are
available and/or installed, and `brew install` and `brew remove` to
install and uninstall a package.  There's currently no support for
custom taps or anything other fancy Homebrew features.
Homebrew packager

We now have a packager implementation that can install packages using
Homebrew.  For now, we don't actually produce any binary package files;
instead we "package" something by copying its staging directory into the
Homebrew cellar, and then "install" it by telling Homebrew to link the
package's cellar into the main Homebrew installation prefix.  Eventually
we might want to look at producing a Homebrew bottle as a binary
artefact.