This has involved a fair amount of work, including:

- Removing Modbus TCP functionality (this will be harder to port)
- Renaming "modbus_tk" module to just "modbus", there is no other
MicroPython Modbus module yet
- Removed use of the "Threading" library, this is not supported on
many MicroPython ports
- Replaced pyserial with machine.UART
- Clearly added LGPL license text
- Added new examples for using the library with MicroPython