Dual Relay Board case and cover

A case and cover for the cheap 2-relay optocoupler boards, like [this one](http://www.sainsmart.com/arduino/arduino-components/sainsmart-omron-5v-relay-opto-couple-for-arduino-uno-mega2560-r3-raspberry-pi-arm.html).
It has mounting flanges, covers the high-voltage parts (and traces under the relays), and also provides strain relief for the wires going to the screw terminals.