FX-79 shelf with damped mount for Pixhawk

Remixed the existing tray to incorporate a vibration damping mount for Pixhawk.

Item is a mashup of these things:

http://www.thingiverse.com/thing:1633693
http://www.thingiverse.com/thing:163472

Pixhawk needs to be rotated by 180' on the yaw axis otherwise the main spar will restrict use of the RC IO pins.