Add OS X VirtualBox builders

Borrowing from [Tim Sutton's template](https://github.com/timsutton/osx-vm-templates/blob/master/packer/template-vbox.json) + some storage hackery = OS X on VirtualBox, with the caveat that VirtualBox's OS X support isn't all that great.

Limitations aside, I've had successful builds of each of the boxes and they all work great with basic Vagrant or Test Kitchen configs.

The `storage_attach` and `storagectl` stuff is only necessary until [this Packer feature](https://github.com/mitchellh/packer/pull/1200) is merged and released. Installing OS X requires a SATA DVD drive, but Packer currently has an IDE one hardcoded (and errors out if it's already gone when it tries to unmount it at the end).