Hook to allow implementation of reconnecting clients

As mentionned in #23, having a ReconnectionConnection[Pool] would be nice when developping daemons and such.
So i made a ReconnectingClient (in doc/reconnecting_client_example.py), which requires a small hook in the _PollingMixin's .continuePolling() method.

This solution may not be the cleanest/most user friendly, but it WorksForMe® and will not break previous code or future code ignoring it.

Feel free to give feedback/rework the code of course.

Regards,