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.
We've considered that and so far I've thought going with regular paths is best since it is the most familiar. People know paths, they know how to use them, how to reason about them, and how to read them. We can use any existing path package here. It seems more cumbersome to add the extra typeness. WDYT?

What I've found in practice is that most people use relative of themselves (e.g.: subcontainer1, subcontainer2) and machine-wide managing daemons use absolute paths.