* factoring of the publishing function
* integration of json publishing to MQTT
* receiving and transcode json messages from mqtt
* replace the TRACE runtime condition by a buildtime one so as to decrease program size
* remove retainFlag parameter not necessary for the moment
* Increase the scope of tests for arduino mega and esp32
* Update arduinojson library to 5.13.3
* add macros to choose between simple or json publishing or both
* differentiate build test for ATmega2560
* correct wrong sprintf definition
* change IDE version for continuous integration
* let default warning level in arduino ide
* correct pins for ESP32
* reorganize user_config
* globally expose repeat functionality as advanced function
* implement repeated sends for Arduino based boards
* use String class instead of C-style arrays to benefit from dynamic allocation and avoid buffer overflows. Try to improve stability of BME280.
* replace sprintf with String class to avoid buffer overruns by dynamic allocation
* fix caps on variable
* re-add TSL2561 code that was accidentally removed
* Add support for TSL2561 based light intensity measurement
* replace Serial prints with trc, add TSL2561 code to test config
* add Adafruit TSL2561 lib to Travis config
* avoid multiple definitions and fix missing string constructor in trace
Florian