Add support for LISTEN/NOTIFY.

Using db.Query("LISTEN relname"), returning an
infinite Rows object. This is not optimal as a
Rows.Next() call cannot be interrupted, but should
fit the API well.
Merging the proposed LISTEN/NOTIFY support into this branch

Note that the current approach is probably *not* a good idea;
(see discussion at https://github.com/lib/pq/pull/106), but
it's useful enough to move forward with.