Add OemGatewaySerialListener

OemGatewaySerialListener is a simple listener that monitors a serial port
for data of the form 'NodeID Val1 Val2 ... \r\n'.

Code refactoring:
- OemGatewayRFM2PiListener inherits OemGatewaySerialListener.
- Processing of the generic data form defined in OemGatewayListener.