Update Stepper.cpp

This is a back-compliant version of the Stepper library which supports the half wave stepping.

1) 'number_of_steps' member has been renamed 'steps_per_turn'
2) added the member 'steps_left' and the relative method to know wether the motor is running or not
3) added the method 'position' which reports 'step_number'.
4) added 'set_step' and 'loop' methods not ot hang up the CPU (as 'step' method does). This allows the CPU to perform other tasks
while the motor is moving.