Greg's Wade reloaded

In this remix of Jonas Kuehlings version of Greg's Wade Extruder I made the following changes and additions:

* added illustrated [instructions](http://www.thingiverse.com/download:1602512) (pdf with vector graphics)
* fixed 1.75mm version
* added printable rod for the idler (untested!)
* I only provide a 1.75mm and a 3mm STL file (both without hotend mount) - all other variations can be created with openSCAD

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Changelog:

* 2017-02-16:
* added "hotend mount" for M6 s