[PyGreSQL] Composite primary keys - again

[Christoph Zwerschke]

D'Arcy J.M. Cain wrote:
 > I always add "SET DEFAULT_WITH_OIDS=ON" at the start of every schema
 > definition file.  I was thinking that we should probably change it to
 > use the primary key if available (and perhaps only if not complex) and
 > fall back to OID only if we can't find one.

I think that's a good idea and another reason why we should make 
PyGreSQL cope with composite primary keys.

-- Christoph