Atmega 3260 Compatibility

The code is currently only working on an Atmega 328P (timers, port names, external interrupts).

It should work with an Atmega 3260 chip as well ([detect the microcontroller at compile time](https://electronics.stackexchange.com/questions/58386/how-can-i-detect-which-arduino-board-or-which-controller-in-software)).