2X Extruder Upgrade

This is a remix of 256754, and credit should go to the author of that thing.

Here in the UK we can't get the recommended spring from the US suppliers (at least not without jumping thru hoops!), and whilst other alternatives are available this modification provides a means of applying pressure to the filament via small M4 bolts.

If you need springs for this design take a look at the following:

http://e3d-online.com/RepRap-Spares-Mechanical/Compression-spring-21x7.5mm

This update has i