Add optional release directory argument to verify_gpg_sigs
Add optional release directory argument to verify_gpg_sigs

- The release directory can now be optionally passed to verify_gpg_sigs
  so the script can be run from a different directory.
- Update with usage example.

MJJoyce/apachestuff,MJJoyce/apachestuff,chrismattmann/apachestuff,chrismattmann/apachestuff