AvrClock: Implement PWM with 16-bit timers.
aprinter-ramps13.cpp: Use hardware PWM where appropriate.
This includes moving the second fan from MegaPin5 to MegaPin11.
aprinter-melzi.cpp: Remove documentation.
It is out of date, and important stuff is supposed to be in the readme already.
aprinter-melzi.cpp: Use hardware PWM where appropriate.