Remove filesystem implementation bias from API

Bring the API into line with the IRC discussion, July 17.

There seemed to be a consensus that the API should be dealing with individual containers and collecting together containers and allowing access from other processes should be the responsibility of the caller.

- Remove `Path()` and add `Id()` in `Container` interface. Update description.
- Remove `Load()` and modify `Create()` in `Factory` interface to remove references to path.
    Update description of `Create()`.

The `libcontainer` API deals only with individual `Container` instances, and does not
provide a 'reaccess' method. `Id()` is exposed for diagnostic information.

Docker-DCO-1.1-Signed-off-by: Steven Powell <spowell@gopivotal.com> (github: Zteve)
I know @crosbymichael disliked the memento name, given that State and Checkpoint are already taken. I think memento it is :)