don't create a history entry when an object in a backref has changed

The code that determines whether an object in a relation has been added/removed does not take into account that that relation may be a backref. If the relation is a backref, then nothing on the current table is changing, and therefore no history entry should be created.
Merged in goodscloud/sqlalchemy (pull request #2)

don't create a history entry when an object in a backref has changed