Added support for postgesql listen and notfiy

As libpq doesn't support callbacks on a NOTIFY, I added two ways to check for a notify in order to check immediately (checkNotify) or with an idling-until-something-on-the-socket happens. 

I'm not really sure, if the checkNotifyIdle is the best solution for others, but in my application I just create a thread, which runs that function on and on and calls a callback-function if a NOTIFY is retrieved.

I'm also not bound to the Notification struct if you think it's a bad idea.