core: ZHA Illumination value parsed incorrectly

Home Assistant release with the issue: 0.91.4

Last working Home Assistant release (if known):

Operating environment (Hass.io/Docker/Windows/etc.): Hass.io

Component/platform: ZHA

Description of problem: ZHA does not parse the Lux illumination values correctly from the sensor. The Lux value displayed is the raw ZigBee value and not the converted one.

Problem-relevant configuration.yaml entries and (fill out even if it seems unimportant):

Traceback (if applicable):

Additional information: The conversion as specified in the ZigBee HA documentation is MeasuredValue = 10000 x log10 Illuminance + 1, so to convert back to Lux it would be Illuminance = 10^((MeasuredValue - 1) / 10000). If MeasuredValue is zero then that means the value is too low to be measured, so the result should probably be zero too.