fix: Reconnect failure

[aragaer]

The "notified on disconnect" flag prevents double disconnects but once
it is set it is not being reset. So it does prevent double disconnect
event on first disconnect but on second disconnect it doesn't fire
"disconnected" event at all. This fix resets the "notified on
disconnect" flag before attempting new connection.

closes issue #294

[aragaer]

This patch works with localhost connection but it seems it actually performs double reconnect when I'm using my custom transport. In this case I get the following:

• heartbeat missed
• receiver loop ends
• notify("receiver_loop_completed") is called
  • "notified on disconnect" is set
  • on_disconnected method is called
  • reconnect happens
  • "notified on disconnect" is reset
• "notified on disconnect" now is false again, so notify("disconnected") is called again

Looks like the better solution would be to have something like this instead:

logging.info("Receiver loop ended")
self.notify("receiver_loop_completed")
if notify_disconnected and not self.__notified_on_disconnect:
    self.notify("disconnected")
self.__notified_on_disconnect = False