Prusa bed leveling sensor for use with dual fan bracket for E3D

Summary for use with dual fan bracket for E3D v6 : http://www.thingiverse.com/thing:1500286 and proximity sensor for bed leveling/tramming. The bracket is modified a bit as well because the heater block was touching the ABS fanduct in the original design (holes have been moved 3mm and the bracket has been made thickker to make the needed room.