Remove gear client in favor of docker - fixes 712

Fixes #712.

* Sorry it's so big. Mostly it's just shuffling stuff around and renaming variables and attributes, actual changes are quite small: adding ``--volumes-from`` equivalent to `DockerClient`, and having `Deployer` create a `DockerClient` instead of a `GearClient`.
* While this uses `DockerClient`, a bunch of references to gear (in module names, etc.) still remain; I will do those in follow up branch so this doesn't get even bigger.

DO NOT MERGE UNTIL 0.2 HAS BEEN FORKED. This will lead to some amount of instability.
This is probably related to #234.