Make Iterator a daemon for proper exit

[jochasinga]

As per this issue I have added self.daemon = True to Iterator's constructor thus upon the termination of the main thread the daemon thread will die.

>>> it = util.Iterator(board)
>>> it.start()
>>> it.is_alive()    # -> True
>>> quit()           # Used to hang, now terminates

If a test case is required for this, I'll write that and issue in another request.

The entire Python program exits when no alive non-daemon threads are left.
https://docs.python.org/3/library/threading.html#threading.Thread.daemon

[edne]

Please merge this, or implement something equivalent

[jochasinga]

@edne according to the last commit in 2015, it seems like @tino isn't active on this project anymore. You can use my fork if you need this fix now.

[tino]

I have indeed been quite inactive. Just found some time to merge some PR's though!