Add `ActivatorQuerySet` and `ActivatorModel` tests

I've been having a problem with not being able to chain `active` and `inactive` calls using the `ActivatorModel`, so I've added `ActivatorQuerySet` to allow for this, and I've added tests for `ActivatorModel`. All tests passing and flake8 runs clean.

I would also love to help getting the `activate_date` and `deactivate_date` stuff working. If someone has something in particular envisioned for this, please let me know, otherwise I'm happy to propose something.

Thanks!