Extra clearance for x-end-motor part for Prusa i3

Those of you that get trouble with X axis motor crashing into the RAMPS board mounted on Prusa i3 frame are not alone. I did have the same problem and decided to fix it by adding some additional clearance to the part.

Source code is added (it should be added as original part was GPL'd) so you can chose the clearance you need. Two sample parts are included too.