Fix many to many multiple add

The two added tests test_adding_multiple_related_objects and test_querying_multiple_related_objects_on_added_object were both failing,
this pull request fixes them.

This pull request includes the code included in pull request #17 .

I think we should also add two tests for "add_at": 
- one within the range of validity of the current benny object
- one outside (prior to) the validity range of the current benny object