Add support for TSL2561 based light intensity measurement (#150)

* 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

OpenMQTTGateway.ino

@@ -189,6 +189,9 @@ void setup()
   #ifdef ZsensorBH1750
     setupZsensorBH1750();
   #endif
+  #ifdef ZsensorTSL2561
+    setupZsensorTSL2561();
+  #endif
   #ifdef ZgatewayIR
     setupIR();
   #endif
@@ -275,6 +278,9 @@ void loop()
     #ifdef ZsensorBH1750
       MeasureLightIntensity(); //Addon to measure Light Intensity with a BH1750
     #endif
+    #ifdef ZsensorTSL2561
+      MeasureLightIntensityTSL2561(); // Addon to measure Light Intensity with a TSL2561
+    #endif
     #ifdef ZsensorDHT
       MeasureTempAndHum(); //Addon to measure the temperature with a DHT
     #endif