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.
Wouldn't quite call it wrong; they're just passing straight through to `execve` and don't want to make any assumptions about order not mattering.

Will no interpolation ever occur in the configured env? I could see 'PATH=/foo/bin:$PATH' being pretty common, but I guess either precomputing it upstream, or forcing users to evaluate bash scripts instead is fine. If interpolation is ever planned, you'll want a []string, so the variables can refer to each other. `os` provides this helper function:

http://golang.org/pkg/os/#Expand

If none of this matters, then sure.