Changed docker.kill() to docker.rm()

Since we actually want to destroy the container, so we need
to delete the container after killing it. We can do this by
calling docker.rm(), which by default does a 'docker rm -f'.
Merge pull request #5 from yifan-gu/destroy

Changed docker.kill() to docker.rm()
Merge pull request #4 from yifan-gu/usage

Added usage() for the docker containerizer.
Added resources allocation in usage.