modbus over UDP

[Ladislav Sopko]

Hello.

I found one old attempt to make UDP transport in this library: Old Attempt

but it was not working. So I take it and finish it. 

Now it is working (at least in my tests, I'll do more development with it in next days.)

If anyone is interested in: Working modbus over UDP.

If you want I can make a merge request. 

Anyway if some one need it, it is there, any testing is highly appresciated :)

Thx

Ladislav