*: purge offer/bid code
Merge pull request #761 from bcwaldon/purge-offering-code

*: purge offer/bid code
client/http: add new Unit methods

This builds on #756 and introduces parity in the HTTP client's
methods with those introduced to the registry client.

The current implementation is very dumb in that it simply wraps Jobs()
for all three methods.

This also exposed a small bug in the client in that the target state was
not being set correctly.
Merge pull request #759 from jonboulle/jobswork

client/http: add new Unit methods
api: use newUnit helper in tests
registry: drop hash-comparison safety

Leave comparison up to the calling code for now.
api: drop unit hash comparison in DELETE
api: drop unit hash comparison in PUT
unit: use go-systemd Option as canonical format