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.