Major rewrite + improvements in many areas

[mbs38]

Hi, first off: Of course I do not expect this to be merged.

My little rewrite has brought some improvements:

• more abstraction when writing to coils/registers using mqtt. Writing is now possible without having to know slave id, reference, function code etc.
• specific coils/registers can be made read only
• multiple slave devices on one bus are now fully supported
• polling speed has been increased sgnificantly. With modbus RTU @ 38400 baud more than 80 transactions per second have been achieved.
• switched over to pymodbus which is in active development

I've made so many changes that compatibility is broken BUT my changes might be very useful considering the issues of modbus2mqtt.

So if you're interested: https://github.com/mbs38/spicierModbus2mqtt

But beware: this is a beta or maybe even an alpha version Can be considered stable now.