Add initial implementation of linux container

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@docker.com> (github: crosbymichael)
Only argument against this is that State() has a lot more information than RunState() which may be queried more often (for when the container does not exist for example).

Also, do we want to expose the full state in the API? I see value for PID, PID start time, but maybe not for things we want to keep private? At the same time, there may not be anything to keep private.