v0.6: Hookbot support, integrate git builder

Merged PRs:

- #35 - Add CONTRIBUTING.md
- #36 - Update FROM statements to 'ubuntu'
- #37 - Update JSONMessage import path
- #38 - Pickup iptables if in $PWD
- #39 - Add daemon mode
- #40 - Add hookbot support
- #41 - Refactor ImageSource and container Flip
- #43 - Implement `hanoverd builder`