OS X guest template

To close #481. Renamed it to the generic 'OSX' as it's not version-specific. The prep script supports building both 10.7 and 10.8 guests on 10.7 and 10.8 hosts.

The built image is about 5.5 GB compressed which is hefty, but in the future one could include a sample postinstall script to remove some unnecessary pieces (depending on what someone needs). It's also sort of possible to remove some items from the initial installation, as well but that's more work in the prep script and so less modular. I'm already regretting having written in the prep script in shell :frowning: 

Thanks @jedi4ever for the guidance and for VeeWee!