C920 Prusabox mount.

I needed to upgrade from by C270 to the C920 webcam for my prusabox. There was one by @wilsondg_201245 but I couldn't get a good angle looking down. So I created a new design where I can have more tilt on the z axis. To keep the camera from slipping, I added a tension screw on top of the holder.

You need a couple of M3Ns square nuts, two M3N nuts, an M3-20 screw and three M3-10 screws.