Can the ActivityPlugin automatically track changes?#362

In the `tests` directory, all the inserts to `Activity` are done manually. For example: https://github.com/kvesteri/sqlalchemy-continuum/blob/master/tests/plugins/test_activity.py#L87 ```python activity = versioning_manager.activity_cls( object=article, verb=u'delete', ) ``` This is in contrast to the `%s_version` tables, where inserts occur automatically. By 'automatically', I mean that the library handles inserts. Anyone using this library does not have to write insert statements. Why is this? Can inserts to `Activity` occur automatically? If not, what is the intended use case?