Prusa i3 MK2 Clone Extruder for 8/12/18 mm sensors and MK8 Hobbed Pulley

[Please show your appreciation of my work by sending a beer token](https://paypal.me/geoffsim59/1 "PayPal") Modified Prusa MK2 extruder parts to fit 8, 12 and 18mm sensors, and the MK8 hobbed pulley. 12 July 2017: 12 and 18mm sensor mountings have been modified to provide more space for the sensor cable routing. I have included the modified scad files, as required by the GPL license. Note that these files may require additional files, which can be found in the Prusa GIT repository. [Please show your appreciation of my work by sending a beer token](https://paypal.me/geoffsim59/1 "PayPal")