SQLAlchemy fixes

This pull request fixes two bugs I encountered when using the SQLAlchemy orm.

1. The disconnect method doesn't commit the session after removing the association, preventing the association from being really deleted.
2. Calling the **remove** method of a ```SQLAlchemyAssociationMixin``` raise an ```InvalidRequestError: Could not evaluate current criteria in Python. Specify 'fetch' or False for the synchronize_session parameter.```