Add notice receiver/processor bindings

By default postgres writes `NOTICE`s to stderr, this exposes the functions to set a receiver and processor function (per the docs, most people will only overwrite one and do everything in that).

If you merge this, I think postgresql-simple should call disableNoticeReporting by default when connecting - most libraries seem to do this. Right now my test logs are getting inundated with `NOTICE`s from `TRUNCATE… CASCADE` statements running in between tests. I can make a pull request for that as well.

I'm not sure if I should be calling `freeHaskellFunPtr` after importing the old processor/receiver functions, or if returning those is valuable at all. Advice?