Cloud Foundry libcontainer API proposal

This builds on our [minimal changes](https://github.com/cf-guardian/libcontainer/commit/2575fbf353f9fab66c94fc476bec59805f8c288b) to [@vmarmol's API proposal](https://github.com/cf-guardian/libcontainer/commit/62e19761a077b3d5dfdd056df06da771b3a377ba).

This proposal is more ambitious and attempts to unify Containers with and without namespaces. A further possibility, not covered here, is to extract the non-namespaced "container" function as a "process in a control group" primitive.
Typically Env is represented as `[]string` of "K=V", as in `exec.Cmd`:

http://golang.org/pkg/os/exec/#Cmd

Doesn't really matter, as I'm assuming no Bash-style interpolation happens so the order is insignificant, but sticking with conventional Go types everywhere helps the API sink in.