Add mesos 0.19.0 containerization support

These two commits add support for scheduling jobs with a container specification as was added in mesos 0.19. I tested it against Mesos 0.19.0 and Deimos 0.3.4. Example:

```
curl -L -H 'Content-Type: application/json' -X POST -d '{"schedule": "R/2014-07-04T16:00:43.000Z/PT24H", "name": "container_job5", "epsilon":"PT15M", "command": "", "owner": "cdegroot@example.com", "async": false, "container": {"image": "docker://registry.internal:5000/hello-batch"}}' chronos:8081/scheduler/iso8601
```