Allow overriding records manager class in register

By calling register(Model, records_class=Foo), a user can choose a
different class than HistoricialRecords to use for the manager.  A silly
example is included for tests.  A more serious example is at:

https://github.com/jwhitlock/web-platform-compat/blob/master/webplatformcompat/models.py#L38
Change style of setting records_class default
Merge pull request #135 from jwhitlock/override_manager_in_register

Allow overriding records manager class in register