Basic implementation of hardware PWM for Arduino Uno

Basic implementation of the PWM functionality for Arduino Uno (Atmega328P) using the 32kHz crystal oscillator as clock source.
The default PWM clock is 256Hz, resulting from a 128 prescaling factor.
The default duty cycle is 50%.
Minor edits in the _qk_adc_p.h_ and _qk_adc.h_ headers