Feature/json-opt

Saves 252 bytes progmem. (1 byte increase in data memory.) Fixes a typo in printTemperatureJSON.

Before

Program Memory Usage 	:	25798 bytes   78.7 % Full
Data Memory Usage 		:	971 bytes   47.4 % Full

After

Program Memory Usage 	:	25546 bytes   78.0 % Full
Data Memory Usage 		:	972 bytes   47.5 % Full