Start running pre-set function every interval seconds.

def start(self, interval, now=True):
        """Start running pre-set function every interval seconds.
        """
        if interval < 0:
            raise ValueError('interval must be >= 0')

        if self._running:
            self.stop()

        self._running = True
        self._interval = interval
        if now:
            self._self_thread = hub.spawn_after(0, self)
        else:
            self._self_thread = hub.spawn_after(self._interval, self)