ModbusPal v1.6b - RC release

This release of ModbusPal v1.6b is a release candidate version.

Fixed bugs:
- Would crash at startup if RXTX not installed

Added features:
- Support for 1, 1.5 or 3 stop bits (serial link)

Known limitations:
- "Tuning" features are not documented yet
- "Chart" feature is not documented yet
- Javadoc very incomplete