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.
Will split this update into multiple parts. It is tricky that this doesn't become a call per knob. WDYT of having more specific (but not just one knob) categories? For now I only have Cgroup, I can see us adding for Network, Mount, and Devices.

We can also take the approach of updates being DIFFs where we only update what is specified. Getting the actual spec can be done with the Spec() call so no remembering should be necessary (but I also tend to like DIFF updates more, as they are almost always the most common case).