Heatbed cabling strain relief

**Update 28/08/2017 I recently printed a rear mounted cooler fan on the extruder assembly and realised the strain relief was catching the new fan assembly. So I redesigned the the armatures to provide a lower profile and keep the assembly below the height of heat bed. Additionally tweaked the cable clamp so it interlocks, all new parts are labelled v4, the purple printed parts in the images show the new bits. I have removed the older files as these new ones supersede them.**

A common failure with Prusa i3's is the heatbed cable breaking over time, pending on the heatbed there are strain relief options out there. The ones I found on here all used a heatbed with 2.54mm standard jumper pins, so built my own.

This part is possibly over engineered but I wanted to try and just use printed parts and nice to be able to manufacturer everything. It takes advantage of the lower aluminium plate beneath the heat bed, and clamps a plate assembly firmly into place.

The plate assembly ha