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.
Yep. The DestroyJob error response is not rich enough to tell me if this was the user's fault or a backend problem. I'll probably need to fix that before I can return a sensible 4-vs 500 level code here.