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 probably want an accessor here and memoize it to only do the work once.

```ruby
def candidate_version
  @candidate_version ||= generate_candidate_version
end
```

of course you need to be careful with 'nil' and 'false' values, if those are valid return values from candidate_version, you might wind up with something like:

```ruby
def candidate_version
  return @candidate_version if @generated_candidate_version
  @candidate_version = begin
    version = generate_candidate_version
    @generated_candidate_version = true
    version
  end
end
```