[wip] Job model and submission

Allow Jobs to be submitted and enqueued at a `POST /jobs` endpoint, without worrying about persistence for the moment.

 - [ ] Model a Job struct that can be used by the job submission and listing endpoints.
 - [ ] Create a channel in the Context for pending Jobs awaiting execution.
 - [ ] Job submission handler: Authenticate, unmarshal a submitted Job, and add it to the queue, assigning it an ID.