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

Those `#sudo_user` and `#current_user` methods are private, and I couldn't work out great syntax for getting them stubbed without getting an exception about trying to use private methods. This is where I arrived when looking for examples of setting the `ENV` vars. Mainly I wanted to do as little as possible in bringing over the "mixin" code from the cookbook.