i3 direct drive extruder for J-head

This is a component that was designed for RepRap Samuel (http://www.thingiverse.com/thing:489915), that will also fit on the Prusa i3 that has a 30mm mounting on the x-carriage. This is inspired by the K-extruder (http://reprap.org/wiki/K_Extruder), but allows for the proper mounting of 2 springs.