Device sensor: relative humidity

**Is your feature request related to a problem? Please describe.**
My Galaxy S4 has a lot of sensors, including relative humidity, ambient temperature, pressure and illuminance.

Only a few are currently supported. I'm opening this issue for the relative humidity sensor. These phones are very cheap second-hand, so I could even drop a few here and there.

**Describe the solution you'd like**
Support for using the phone's relative humidity sensor in homeassistant.

**Describe alternatives you've considered, if any**
#1235 is an issue about temperature.

**Additional context**
#1235 contains some information on how to implement this. It should be fairly close to the pressure sensor and/or light sensor.