Add ability to have multiple modbus slaves.

[fake-name]

Modbus is pretty much pointless without the ability to communicate with multiple slave devices - You might as well just use a plain old RS-232 connection otherwise.

This adds the ability to change the slave address.

Also, sublime text cleaned up a bunch of trailing spaces.

[fake-name]

This effectively fixes https://github.com/4-20ma/ModbusMaster/issues/109