Remove some mentions of Gear.
Rename GearEnvironment, remove vestigial links references and get rid of
more gear references.
Remove more references to links.
Get rid of Unit.sub_state, making Unit.activation_state documentation
more accurate.
Docker client currently only does active/inactivate states, so don't
bother with anything else.
Get rid of remaining gear references.
Lint fix.
Address review comments: Various docstring fixes.
Merge branch 'final-geard-purge-608' into better-volume-support-in-dockerclient-234
Sketch of changes to Docker client.
Sketch of new alternative to Volume.expose_to_docker.