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

you can use the accessor instead of the ivars (drop the @'s here).  better style and it'll catch typos instead of autocreating a typo'd ivar for you.  you can do that throughout the file.  you should only use ivars directly in initialize and in explicit accessors and setters.  you have to watch out when you convert assignment `@foo = 'thing'` since it needs to go through the setter like `self.foo = thing` and sometimes i leave those as ivars when the thing i'm changing looks like its just a setter in disguise.  there's also cases where initialize should really call the setter and not call the ivar but if you don't have any special logic in the setter its conventional to not call the setter in initialize.