Add stop() method to kill Iterator thread and Exception Handle to KeyboardInterrupt

[jochasinga]

As when creating a new Iterator via

>>> it = util.Iterator(board)
>>> it.start()

There was no way to kill the thread explicitly, and with KeyboardInterrupt the thread hangs. Thus, a method stop() was added:

>>> it.stop()

And self.setDaemon(True) was also added in the constructor of util.Iterator as per this issue.