Merge vores8 Modbus binding changes

I have been using @vores8 vores8/master Modbus binding for couple of months with success (TCP mode).
Fore sure it should be merged into upstream, but there are no pull requests from @vores8.

It fixes #652 - value changed events are only emitted when register value really changes.
It upstream version events are emitted continuously, event without register value change - this is how I understand bug #652.