mb2hal: added functionallity, including arduino example.

- Arduino example added.
  Tested with Arduino Mega 2560 R3 using Modbusino over USB at 115200 bps.
- New parameters in config file.
  HAL_MODULE_NAME, MB_RESPONSE_TIMEOUT_MS and MB_BYTE_TIMEOUT_MS.
- New HAL pin "num_errors" for each transaction. num_errors = 0 if last
  transaction OK, else the count of failed transactions.
- Fix a bug, OPTIONAL's parameters may be 0 instead of default value if
  not specified in 1st transaction.

Signed-off-by: Michael Haberler <git@mah.priv.at>