Improved hot-end platform for 3DR Simple

This is an improved hot-end platform for the [3DR Simple](https://github.com/JohnSL/3DR-Simple-Delta-Printer#3dr-simple-delta-printer) 3D printer.

Features:

* Modeled entirely in OpenSCAD.
* Captive bolts to make tightening easier.
* Compatible with parts designed for the original 3DR micro platform, including fans.
* Compatible with parts designed for the 3DR Simple micro platform, which has a different bolt arrangement.
* Low-profile height (8mm)
* Optional integrated mount