i3 rework hall based auto bed levelling probe

Automaticly retracts, based on the design from [MarcoAntonini](http://www.thingiverse.com/thing:111397) and adapted for prusa i3 rework extruder and mount.
No extra width for the X-carriage, and no servo needed.
I found it to be more consistent then the servo mounted mechanical switch that I previously used.

Parts needed:
* This printed part
* A hall-0 probe (the arrow shaped one, I got mine from [mauk.cc](http://mauk.cc/webshop/cartesio-shop/electronics/hall-o)
* A probe (I use