Added the exec command for Docker 1.3.0

To use, start a container:

    % docker run -it --name shell busybox sh

Then execute a command in the running container using the Python client:

    #!/usr/bin/env python

    import os, sys
    import docker

    cli = docker.Client(os.environ['DOCKER_HOST'], version='1.15')

    name = sys.argv[1]
    cmd = sys.argv[2:]

    print cli.execute(name, cmd, stdout=True, stderr=True, tty=False)

Example:

    % ./exec_test shell ls -1
    bin
    dev
    etc
    home
    lib
    lib64
    ...

I think this shouldn't be hardcoded to `shell` but rather be:

    '/containers/{0}/exec'.format(container))

Or similar. Applying this locally allows exec to work.