Add MAV_CMD_NAV_START_RX_PAIR to COMMAND_LONG enum

- Provide for a better way to start receiver pairing
Move MAV_CMD_START_RX_PAIR to common COMMAND_LONG messages
Merge pull request #136 from jean-m-cyr/master

Add MAV_CMD_NAV_START_RX_PAIR to COMMAND_LONG enum