encoder functionality implemented

[Chaliuz]

This update lets manage the step motor position with a encoder.

[redoxcode]

I am a bit hesitant with this PR since I think when you want to use a closed-loop configuration you should go for a more complex control scheme (PID).
I see the value of using an encoder to compensate lost steps, however I think stuff like acceleration and deceleration should be implemented first to reduce steplos in the first place.
Also I think with your implementation one could run into an issue where the stepper keeps stepping one step back and forth trying to reach the target position indicated by the encoder, if the encoder has a higher resolution than the stepper.

Edit: Here is an publication on that topic: https://www.galil.com/download/whitepapers/wp_closed_loop_stepper_motor_performance_gains.pdf