Add Exec to container API

Closes #69

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@docker.com> (github: crosbymichael)
Because we have to run some type of initialization in the namespace, setns, open console, etc.  and our parent is the one that places us in the cgroup we would end up in an inconsistent state, maybe half of our initialization is done then we are freezed.

I would be much better just to say that when a container is in the paused state no process are running inside the container and you cannot spawn new processes inside.  resume the container then do your exec.