Unassigned job IDs

Bizarrely, it seems that sometimes, the `POST /jobs` call can return a response that [doesn't contain a job ID](https://github.com/cloudpipe/cloudpipe/issues/17#issuecomment-70948605), even when no errors are reported in the logs or the API. The [`JobSubmitHandler`](https://github.com/cloudpipe/cloudpipe/blob/master/api_job.go#L32) should never return without assigning an ID to each job.