Support SQLAlchemy 0.8

[holm]

The method was changed in SQLAlchemy 0.8: http://docs.sqlalchemy.org/en/latest/changelog/changelog_08.html#change-b9d4ab67bbed4b4cbf58ffa3665cc480

[euangoddard]

I will attempt to do a release of these changes on Pypi as soon as possible as there are some other outstanding changes.

[w31rd0]

The attribute name is inserted_primary_key, not primary_key, so this is now broken. See #9