LISTEN / NOTIFY support

Continuation of discussion on #28 

I've implemented direct messaging instead of GenEvent, storing the PID of the caller on `Postgrex.Connection.listen/3` and removing it on `Postgrex.Connection.unlisten/3`.

I've implemented dead-process checking by using `Process.alive?` before sending the notification, but I wonder if there is a better way to do it? I suppose we could delete the process from the HashSet when it's detected to be dead, and when the HashSet size reduces to 0 we can send an UNLISTEN query to the server?
No need for a `Process.alive?/1` check, sending is a NOOP for non-existant processes.