- added Defines
- main(): to distinguish between steering signals and throttle signals,
  the code now checks if the MSB of the received byte is set;
  if yes => steering signal, else throttle signal;
  Attention: the sending code has to take care of setting the MSB to the correct value
- since we now send relative values for steering and throttle, offsets had to be introduced
  and the upper and lower bounds were adjusted
- added debug_pin