API, pt IV: CRUDdy Units

New API calls:
- GET /units/<name>
- POST /units
- DELETE /units

fleetctl is now fully functional if signing & verification is not used. There is still more work to do, but the API can be considered "useful" with this PR.
It's used to look up the IP address of the machine on which a job is loaded/launched in the load/start fleetctl commands.