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.
Derp - I shouldn't have said 500. But yeah, in this case, more granularity in what is bad about the request.