Implement Compare Match Output for timers

[urish]

The Compare Match Output bits are used to generate hardware PWM signals on selected MCU pins. This is also the mechanism used by Arduino's analogWrite method.

From the datasheet (page 113, section 15.9.1):

These bits control the Output Compare pin (OC0A) behavior. If one or both of the COM0A1:0 bits are set, the OC0A output overrides the normal port functionality of the I/O pin it is connected to. However, note that the Data Direction Register (DDR) bit corresponding to the OC0A pin must be set in order to enable the output driver.

[urish]

Some things to note:

• Writing to the TCNT0 Register blocks (removes) the Compare Match on the following timer clock.
• Force Output Compare bits: Forcing compare match will not set the OCF0x Flag or reload/clear the timer, but the OC0x pin will be updated as if a real compare match had occurred (the COM0x1:0 bits settings define whether the OC0x pin is set, cleared or toggled)