Refactor DockerSensor to expose two triggers

- docker.container_tracker.started - dispatched when a new container is started
- docker.container_tracker.stopped - dispatched when an existing container is
  stopped
Don't allow user to specify "all" argument and default it to "False". If we default it to "True", stopped / deleted container will never be detected since "ps" will also return stopped container.
Remove unused variable and shebang.
Update docker pack readme.
Merge pull request #50 from StackStorm/docker_sensor_trigger_refactor

Docker sensor trigger refactor