Added the 'stop' command (with the timeout parameter before resorting to 'kill').

Fixed broken tearDown in DefaultDockerClientTest as it wasn't removing containers.
Merge pull request #31 from tedyoung/feature-add-stop-command

Added the 'stop' command (with timeout parameter before killing)
Added method for retrieving list of images

You can specify parameters to list all images,
or dangling images.