Changed the test. Instead of driving a single buzzer it now controls 2 LEDs.
The buzzer test is now in a different tester.
Fixed pwm: now all the channels work independently!
Also split the set_channel_value to 2 different functions:
    * set_channel_dutycycle to set the duty_cycle on the channel, and
    * set_max_value, to set the max.
Fixed Buzzer to use the new PWM interface.
It now also has a new ctor parameter that's used to set the duty cycle, by default it's set to 50(%).
Fixed the pwm tester to use the new PWM interface.
Adding a new tester for pwm/buzzer/timer: it uses two timers,
 - one to toggle 2 leds using pwm
 - the other one to play the melody.
all 3 active components are mapped to 3 channels of the single pwm that uses timer2.