Implement calibration of the servo

[ffuubarbuzz]

When I built my device, original values (1176/1400/1677) didn't work for me, I had to "calibrate" and change the value in the firmware to 1176/1490/1757. Would be nice to be able to calibrate it on-device without a need to change the firmware. Depends on #11.

Maybe I missed something when assembling the device though, in that case feel free to close this issue.

[CoretechR]

Good idea! It's normal that you have adjust or at least to fine tune the servo values. Hobby servos are not really meant to have perfect zero positions. And you can attach the servo horn in different angles.
This would be one more application for persistent settings.