Added the ability to set the digital relays on the APM

Adding the ability to use the message:

181:	MAV_CMD_DO_SET_RELAY
Set a relay to a condition.

Mission Param #1	Relay number
Mission Param #2	Setting (1=on, 0=off, others possible depending on system hardware)
Mission Param #3	Empty
Mission Param #4	Empty
Mission Param #5	Empty
Mission Param #6	Empty
Mission Param #7	Empty