Add support for PostgresSQL set_notice_receiver

By adding a notice receiver to the connection one can send notices from stored procedures to a log file instead of stderr, which is what we use it for. I thought this might be a generally useful feature to expose since it is so little code.

I did not manage to the postgres and postgres-pr gems to work on my computer, and since they are unmaintained I decided to skip adding notice receiver support for them. Is that ok?