----------------
Bugs fixed:
* New objects added to a session did not cause a transaction join, so were not
committed at the end of the transaction unless the database was accessed.
SQLAlchemy 0.4.7 or 0.5beta3 now required.
Feature changes:
* For correctness and consistency with ZODB, renamed the function 'invalidate'
to 'mark_changed' and the status 'invalidated' to 'changed'.