Adding RunIn to run a user specified command in an existing container.

This is aimed at providing RunIn support in docker. I don't personally like the API as-is since it requires the users to provide container specific information which is already checkpointed. In the future, the API will evolve to just require the container ID and derive the config and state based on it. 
What about factoring out the code that builds the args since it is shared with ExecIn?