Fix #2 Work around HardwareSerial for SAM3 micro
Fix #2 Work around HardwareSerial for SAM3 micro

- add #ifdefs to check for presence of AVR or ARM chipset (use HardwareSerial
  for AVR and UARTClass for ARM)
- change MBSerial to pointer and update all references to use -> notation
  instead of . notation

4-20ma/ModbusMaster,kintel/ModbusMaster,4-20ma/ModbusMaster,4-20ma/ModbusMaster,kintel/ModbusMaster,kintel/ModbusMaster,kintel/ModbusMaster