Initial version of libcontainer API.

Docker-DCO-1.1-Signed-off-by: Victor Marmol <vmarmol@google.com> (github: vmarmol)

This is an initial proposal of what a libcontainer API would look like. You can see it is in no way implemented :) This is primarily for discussion on the API, we can merge this and start working on the individual pieces one at a time.

The proposed API borrows heavily from lmctfy and libct with some changes to improve on those. Key aspects are:
- Container names as hierarchical path
- Separating Creating a container from Getting a container (the latter gets a reference to a running container)
- Containers as objects we can perform action on.

Any and all comments are welcomed :) I think the documentation on these may be a little lighter than I'd like. I can add more as it is requested.
@bernerdschaefer added a return to Start(). That was an omission on my part.

@xemul, I think the case would be the same as if the init died (or exited) for whatever reason. It is certainly a case we need to handle well. I don't think it is something that will "spoil" the state of the container as we can see that the PID is no longer alive.