added 'entity_name' keyword argument to mapper. a mapper is now associated with...

added 'entity_name' keyword argument to mapper.  a mapper is now associated with a class via
the class object as well as the optional entity_name parameter, which is a string defaulting to None.
any number of primary mappers can be created for a class, qualified by the entity name.  instances of those classes
will issue all of their load and save operations through their entity_name-qualified mapper, and maintain separate identity from an otherwise equilvalent object.