Z-Wave THERMOSTAT_SETPOINT command class implementation

Actually I didn't do much on this, mgbowman did all the heavy lifting and I thank him for that! That's what keeps the Z-Wave binding going forward.

Now I've added the supported_get methods and implemented variable length bigdecimal encoding. Since I have my thermostat hardware not connected yet, I need someone else to test these adjustments before or after merge.

If any one would be able to pick up these changes and test them I'd be eternally thankful. Way better than me having to drain my entire central heating system, replace the thermostats and filling it up again before I can test this code.