In model.to_dict(), skip field if not model descriptor.

Skipping '_sa*' fields isn't enough. Assocation proxies, e.g., start
with '_AssociationProxy*'. So instead of playing whack-a-mole, just
limit to actual descriptors.
In model.to_dict(), if value has it's own to_dict() function, call it.
Add to_dict() hook for EnumSymbol.
Release v0.4.2.