Micro DUAL/QUAD Extruder

##Number 3 of 5 for [Gyrobot's Extruder Week](http://3dprintboard.com/showthread.php?3038-Single-Dual-and-Quad-Extruder-Week).
This is a dual extruder with a small footprint (105mm x 42mm x 42mm). Two can be placed back to back to create a quad (105mm x 84mm x 42mm). I got a friend to measure the size of a Replicator 2 single extruder (not dual) which is 71mm x 84mm x 76mm so it seems even the Quad has a smaller volume.

The fact that it is small, increases the possibilities to print with dual hotends where space or weight may be a factor. The pitch between the hotends is only 20mm for dual and quad.

I know Bowden extruders are one option where space is limited but I designed this with the requirement to print with FFX elastic filaments from [Recreus](http://www.recreus.com/), something that Bowden extruders can have issues with.

- It can be mounted onto multiple types of carriages either in X or Y orientations.
- Cooling of the hotends is provided from