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).