Add Exec to container API

Closes #69

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@docker.com> (github: crosbymichael)
I don't think we should reparent.  When the init of the container exists these processes are still killed because the namespaces will be gone when PID 1 dies.  We don't want to have some time of long running init in the container ( via the libcontainer API, if you want this there is nothing stopping you ) so we cannot reparent.  This was just a comment that was left over.