Prusa MK2 RPI Zero Camera mount

[A new, improved version is available here](https://www.thingiverse.com/thing:2486818). This new holder is fix and it is made for the default Pi Zero W case.

[Prusa Mk2 mounter by Aero3D](http://www.thingiverse.com/thing:2185916) is great, but I had a couple of problems:
1) the mounter does not allow the connect the rpi zero with the printer via usb;
2) the rpi hit the fan when the axis are set to home.
With this remix (I used the original file, and reworked with openscad) it should be fine, I will upload photos as soon as I take them.