Added/tested "get_or_create" to return a unique object from the database.

Use:
>>> x = orm.get_or_create(orm.Thing, name="Rumplestiltskin")

If a unique "Thing" in the database has this "name", the thing is returned.
Otherwise, a unique Thing with is name is created, added to the database, and
returned.  Fails if name is common to more than one Thing in the database.