Allow drivers to add methods to dbh and stmt objects
(note that we can't use a class, because the use only sees the PDO class).
Clarify the api slightly:
PDO::exec() is used for one-shot queries that don't return rows
PDO::query() is a convenience function for returning a rowset without
having to go through the steps of preparing and executing.
Still more to come. Give it a couple of days before starting to write drivers
for the other databases; a few things might change, so I'd like to coordinate
that, but in a couple of days.
