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.
Made the name changes since I think they make sense. The semantics of the API today allow for creating with and without namespaces. That is important today for many users I believe. The details of how we implement systemd for cgroup-only containers is still a bit unclear so I want to make sure we don't block on it, but have flexibility if we have to go one way or another. I think the API as is defined by @rjnagal allows that.