Replace obsolescent prog_char for PROGMEM

This is needed with avr-gcc>=1.8 [0].

[0] http://www.tuxgraphics.org/electronics/201207/prog_char.shtml

Signed-off-by: Olivier Mehani <shtrom@ssji.net>
Include Arduino.h instead of WConstants.h

Need for Arduino>=1.0 [0].

[0] http://arduino.cc/forum/index.php/topic,85529.0.html

Signed-off-by: Olivier Mehani <shtrom@ssji.net>