Get a message from the bus, waiting up to the specified timeout.
If timeout is 0, this function behaves like Bus.pop. If timeout is
GST_CLOCK_TIME_NONE, this function will block forever until a message was
posted on the bus.
the gstreamer.Message that is on the
bus after the specified timeout or NULL if the bus is empty
after the timeout expired. The message is taken from the bus
and needs to be unreffed with Message.unref after usage.