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

I'd probably do this way up at the top of this file:

```ruby
let(:resource_version) { '24.3' }
let(:new_resource) do
  new_resouce = Chef::Resource::HomebrewPackage.new('emacs')
  new_resource.version(resource_version)
  new_resource
end
```

then:

```ruby
let(:resource_version) { '24' }
```

when i needed it.

that reduces the stubbing, but tends to generate a lot more nested context's though.