Multipwm

Multipwm is now finised ! Finally !
There is a test that controls three servomotors in `test/multipwm`.
It is supposed to work with 8 but, I don't have got 8 servomotors.
You can, anyway, try to compile the test for any AVR.
It doesn't compile for SASIAE, since interrupts are used (and it is also not implemented fo SASIAE).

EDIT by Elioty: This PR answer #24.