Add namedtuples() execution method to Postgres.Statement

[jwp]

I think this may be the best route for supporting native Python objects from Statements.
Rather than using a statement wrapper, just have the Statement.namedtuples() convert. It doesn't make much sense to pack Postgres types into as rows() suffices well enough.

Statement.namedtuples() -> iterator
Statement.namedtuple() -> next(Statement.namedtuples(...)) or None