Add ability to nest Configs.

Add ability to nest Configs.

I also restructured the storage backends as distinct helper classes
rather than Config subclasses.  This means that you can use several
Storage instances to save a single Config instance (although each
Config instance will have a single default Storage instance stored in
config._storage).  This avoids all the automatic subclassing
shenanigans, and makes loading and saving from different backends much
easier.