Added 'bool' return value to Containerizer::launch.
Added composing containerizer and --containerizers flag.
Added a 'Docker' abstraction.
Added a Docker containerizer.
Bug fixes and cleanups in Docker abstraction.
Integrated DockerContainerizer.
Updated an example to use ContainerInfo to run Docker.
Add environment test for docker
Added pid() in docker/docker.cpp to get the pid of the container.
add ps option
Added rm() in docker.hpp/cpp to enable users remove containers.
Modify validate docker to call info instead
Added docker unit test, docker flag and fix issues found
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()
Added usage() for the docker containerizer.
Added one unit test for this.
Merge pull request #4 from yifan-gu/usage

Added usage() for the docker containerizer.
Display resources allocation in ResourceStatistics.
Support resources in docker run().
Refactored MockDockerContainerizer.
Merge pull request #6 from yifan-gu/usage

Support resources in docker.run(). Display resources allocation in usage()