dockerhub webhook support

This enables auto-deployment from webhooks sent from the Docker Hub.

# Workflow
* Code is committed / pushed to repository (GitHub/Bitbucket)
* Docker Hub builds image
* Webhook is sent to Shipyard
* If build is a success, pull and restart containers for that repository

# Limitations
* Will re-deploy all containers that are using the image (ignores tags)

# Status
- Depends on citadel/citadel#68
- Testing can be done against `shipyard/shipyard:test`