Fix genty being used on a parent class and base class defined in separate files and run with a test runner like nose.

Genty needs to not collect tests that have already been genty_generated to avoid expanding already expanded tests.
Fix linting error from new tox/pylint.
Add test coverage for subclass + metaclass fix.
Merge pull request #13 from box/fix_subclasses

Fix @genty decorating subclasses and base classes