[SQLObject] Expanding classRegistry

I mentioned this a while back, but it's now upon me, and I need a way
to create distinct classRegistry's within the same interpreter
instance, so that I can have classes with the same name (which never
interact).

My proposal: expand classRegistry/needSet to be a dict of
dictionaries, first keyed by a 'registry name' to be optionally set
in the SQLObject classes, and then as usual by class name.

SQLObjects would add another class-level variable, _registry or
_registryName, if not specified then they would go in the default
'__global' name.

Thoughts? I'll quick implement this sometime tomorrow, but would
prefer any yay/nay and naming of variables feedback before I do so.
:)

- Luke