delayIdle
=============
A low-power delay for Arduino

Using the delayIdle() function most common AVR chips consume approx. 1/2 of the power
when compared to the original Arduino delay() function.
  ATmega328p@8MHz@5V   : delay()=13.5mA   delayIdle()=8mA
  ATmega328p@8MHz@3.3V : delay()=3.72mA   delayIdle()=1.35mA
  ATmega328p@1MHz@3.3V : delay()=1.17mA   delayIdle()=0.53mA
Putting the CPU to SLEEP_MODE_IDLE causes some inaccuracy in the time when the CPU returns from the delayIdle() function,
however in most cases the small inaccuracy is acceptable.
The inaccuracy depends mostly on TIMER0 overflow, which depends on CPU frequency : 1ms @ 16MHz, 2ms @ 8MHz, 16ms @ 1MHz