Add SHT11 CoAP Resource and Activate Sensors for CoAP Resources like Light, Battery.

Hi, 

In the er-example-server.c the Light sensor is not activated and as a consequence the reading from the sensor is not right even if the resource is activated. This Pull Request add the lines needed to activate the sensors of the light, battery and radio resources and also add the SHT11 resource to get the temperature and humidity, which might be interesting to have it in the master tree.