Support for special keys to be used in combination in boot_cmd_sequence.

- when a sequence begins with '<Mod>', send the rest of the special
  characters following it together to vnc.key_press, so they are
  sent in combination
- issue #478
Merge remote-tracking branch 'timsutton/vnc-modifier-keys' into feature/osx-guest
OS X template overhauled to fully automated with prep script

- definition using new os_type_id, Veewee::Definition class
- all cmd sequence items commented out, no longer needed
flesh out Darwin ostypes for 10.7/10.8