honor "unit_system" setting for temperature

[magnus919]

Currently the temperature sensor is providing only Celsius readings (understanding that's probably straight from the sensor). Ideally the "unit_system" setting would be honored so that when that is set to "imperial", the right temperature is displayed without every sensor having to be wrapped in templates.

[robmarkcole]

Im not sure if there is a standard way of doing this in Home-Assistant..?

[yottatsa]

If the temperature sensor would be presented as a separate entity, and unit_of_measurement attribute returns either TEMP_CELSIUS or TEMP_FAHRENHEIT, then entity helper should take care of conversion automatically. Depends on #115