- select().as_scalar() will raise an exception if the select does not have

- select().as_scalar() will raise an exception if the select does not have
exactly one expression in its columns clause.
- added "helper exception" to select.type access, generic functions raise
the chance of this happening
- a slight behavioral change to attributes is, del'ing an attribute
does *not* cause the lazyloader of that attribute to fire off again;
the "del" makes the effective value of the attribute "None".  To
re-trigger the "loader" for an attribute, use
session.expire(instance, [attrname]).
- fix ormtutorial for IS NULL