Camera mount for Prusa i3 MK2S bed

This allows a camera (I used a raspberry pi 2.1 one) to be attached to the bed of the Prusa i3 MK2S printer.

**Update holder (v4 19/08/2017):** Made the holder fit tighter, as after weeks of printing it became loose and I needed to insert something between holder and mount for a sufficiently tight fit

I used https://www.thingiverse.com/thing:1568750 as camera frame, this fitted my camera best. When combining it with https://www.thingiverse.com/thing:1646474 as arm attached to the printer,