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.
Mention Homebrew/Mac OS X in README
Update to latest libcork
Parse keg-only `brew info`

There's some extra text that we need to parse in the output for keg-only
formulas.
Set PKG_CONFIG_PATH during Homebrew builds

The autotools and cmake builders now set the `PKG_CONFIG_PATH`
environment variable during the configure step, if the corresponding
`pkgconfig.path` Buzzy variable is set.  On Mac/Homebrew, we
automatically set this Buzzy variable to point into the $PREFIX/opt
directory of each dependency.  Some of these directories might not
exist, but pkgconfig will silently ignore those, so that's not a
problem.
No separate built-package interface

The extra interface for built packages isn't really needed; for native
packages, which we don't need to actually build, we can just use the
noop builder implementation.  So packages are now always created from a
builder instance and a packager instance, and all of the actual
interesting logic happens in those two instances.

This was needed to eliminate a bug where we weren't always installing
build dependencies correctly.  For instance, if package A depends on B,
and B build-depends on C, then we wouldn't install C when trying to
install A.  Now we do.
Add Homebrew formula description

As far as I can tell, Homebrew doesn't support bottles (its version of
binary package files) without having a formula description in the core
Homebrew package repository.  So we can't easily upload binary packages
for Mac like we can for our other platforms.  Instead, this formula
description can be used to build buzzy without having it already
installed.
Merge branch 'feature/homebrew' into develop

* feature/homebrew:
  Add Homebrew formula description
  No separate built-package interface
  Set PKG_CONFIG_PATH during Homebrew builds
  Parse keg-only `brew info`
  Update to latest libcork
  Mention Homebrew/Mac OS X in README
  Homebrew packager
  Install native Homebrew packages