Add support for negative values; remove hardcoded temperature (#30)
Add support for negative values; remove hardcoded temperature (#30)

Negative values are represented in two's complement, which the documented Decoder did not support. 

Also added some documentation, made all examples use the value shown in the existing screenshot (23.48), and removed some workaround that used a hardcoded value rather than the sensor reading.
TheThingsNetwork/workshops,TheThingsNetwork/workshops