Use homebrew for default package provider on OS X

:ice_cream: 

Via [Chef RFC 016](https://github.com/opscode/chef-rfc/blob/master/rfc016-homebrew-osx-package-provider.md), fixes issue #1709, adding the `homebrew_package` resource with the `homebrew` provider, and setting it as the default for Mac OS X platforms (`mac_os_x`, `mac_os_x_server`). This will still require that homebrew is installed on the system somehow, but doesn't require use of the cookbook if users choose to install via some other method (pre-built images, for example).

Example of this in action: https://gist.github.com/jtimberman/1f0a6fd434a6ecef95da

so if candidate_version comes from the parent class and all of the subclasses of Package do this thing setting the ivar then forget about it.  i'll have to ragefix those at some point...