[PATCH] notify support for DBD::Pg

Edmund & all,

Attached is a patch to add support for asynchronous notifications to
DBD::Pg interface.

Added following functions (self-documented in Pg.pm):

$ret = $dbh->func('notifies');

Returns either undef or a reference to two-element array
[ $table, $backend_pid ] of asynchronous notifications received.

$fd = $dbh->func('getfd');

Returns fd of the actual connection to server. Can be used with
select() and func('notifies').

Also modified test.pl to test all of this functionality in the following
sequence (listen, select, notify, select).

Edmund, please consider adding to next release.

Thanks

-alex