Some refactors

Changes:
- Instead of requiring brokers at two IP address, we require URLs (registrar-broker-{1,2})
- Update logic now checks to see if broker exists, instead of catching
  errors
- Usage message reformatted
- New VCR cassettes
Implement broker deregistration

- Deletes broker
- Purges services
Merge pull request #2 from pivotal-cf/deregister

Implement de-register