Allow timer selection for PWM pin attached to several timers

[jfpoilpret]

Some AVR MCU have PWM pins that may be attached to more than one timer (only one at a time of course).
Currently, FastArduino supports only one PWM Timer for a given pin, hence an arbitrary choice is made when support is added for an MCU.

It would be nice to allow timer selection for those PWM pins that are linkable to several timers.

[jfpoilpret]

This enhancement might break API compatibility for PWM feature.
It is likely we need to define a new enum class PWMPin instead of a simple namespace and constants that are just DigitalPin aliases.
Hence, examples and tutorial will need an update.

[jfpoilpret]

Fixed as of commit 80be6fc
Now FastArduino supports PWM pins linked to more than one timer (e.g. on ATmega32u4 or ATmega2560, e.g. on Arduino LEONARDO or MEGA).
Some ATtiny (e.g. ATtiny85) also have some PWM pins linked to 2 timers (0 & 1) but these are not supported yet because ATtinyX5 timer1 PWM is too specific and requires important changes to FastArduino (see issue #20)