- simplified _mapper_registry further. its now just a weakkeydict of mapper->True...

- simplified _mapper_registry further.  its now just a weakkeydict of mapper->True, stores
all mappers including non primaries, and is strictly used for the list of "to compile/dispose".
- all global references are now weak referencing.  if you del a mapped class and any dependent classes,
its mapper and all dependencies fall out of scope.
- attributes.py still had issues which were barely covered by tests.  added way more tests (coverage.py still says 71%, doh)
fixed things, took out unnecessary commit to states.  attribute history is also asserted for ordering.