Better volume support in dockerclient - fixes 234

Fixes #234.

Follow up #289 will add volumes to the `IDockerClient` tests, by including volume info in output of `DockerClient.list` (and higher levels of logic).
This makes me so sad.  I can think of alternatives (inotify!  callbacks!  docker actually telling us when things happen!) but (a) it's not clear whether they're better or merely more complex and (b) it probably doesn't matter.