* Add difference tracking tool
* Improve --diff mode for docker_container.
* Improve diffs of sets by ordering the sets.
* Rewrite imports, get rid of HAS_DOCKER_PY_x variables and use docker_version instead.
* Rename container -> active (more generic).
* Add --diff for docker_volume. Change old diff output.
* Add --diff for docker_network. Change old diff output.
* Add --diff for docker_swarm_service.
* Add changelog.
* Add entry for porting guide on docker_network and docker_volume.
