Commit the session after removing an association.
Pass synchronize_session='fetch' to delete.

When we remove an association, SQLAlchemy throws a InvalidRequestError
exception (see:
http://docs.sqlalchemy.org/en/rel_0_8/orm/query.html#sqlalchemy.orm.query.Query.delete)
Merge pull request #18 from Flyflo/sqlalchemy_fixes

SQLAlchemy fixes