Add Exec to container API

Closes #69

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@docker.com> (github: crosbymichael)
Semantics conditional on the container configuration makes this API pretty messy. It would seem much cleaner either to make libcontainer always responsible for reaping and returning the exit status via the API (our preference) or never responsible for reaping and leave it up to the caller.