====== LEDs und Pushbutton ======

  * http://stackoverflow.com/questions/33833362/how-does-contiki-os-process-external-interrupts
  * http://forum.arduino.cc/index.php?topic=321150.0
  * http://www.atmel.com/Images/Atmel-2549-8-bit-AVR-Microcontroller-ATmega640-1280-1281-2560-2561_datasheet.pdf