Timer 0 and 1 fix on atmega2560

This should fix the bug #29.

Timer 0 and 1 should now work well on atmega2560.
A bug was introduced on timers by the new version of archi files... This is why it would be great to have automatic testing tools !
Also, the timer 1 had no init function instantiated, it is now done in `src/avr/hardware/timer.cpp`.

If you want to test on your arduino, you should go into `tests/timers`, run `make atmega2560`, then `./deploy.sh` and use your favourite program to display serial communication.
The expected output of the program is written at the beginning `main.cpp`.