Modbus binding: Add more serial configurable parameters: datasize, parity and stop bit.

Hi,

I have added few more configurable parameters in openhab.cfg for modbus RTU binding.
Only port and baud was configurable in current version, and I have added data size (7 or 8), parity ("none", "odd" or "even") and stop bit (1 or 2).

So, in openhab.cfg, a config line can look like:
modbus:serial.slave1.connection=/dev/ttyUSB0:19200:8:even:1

Regards.