wordclock thread is now reduced to only taking care of displaying the time;
blink thread and lightsensor thread take care of blinking led and reading
lightsensor (and calculating new brightness value).
* split main loop in two separate threads, using ProtoThreads 1.4 (see
http://arduinocollective.com/2009/12/05/arduino-threading/ and
http://www.sics.se/~adam/pt/)
* implemented simple key repeat delay: first delay is 1 second, after that it is
only 200 ms
* changed indenting (since multithreading was such a big change in it self, it
was relatively painless to do it now)
Requires a phototranistor (TEPT4400) and series resistor (I used 2.2 kOhm) on
ADC0 (pin 23). Range is quite limited though, so will probably need to modify
hardware a bit and do some more software tuning.