Unknown sensors

[mboisson]

This PR adds support for sensors of unknown device class. It also records the units of measurement. This can be used for example to address issue #53, or for any other type of sensor that is not explicitly supported

[ymerj]

Since this pertains to sensors, wouldn't it better to handle de device class definition in the sensor case instead of in the general section? Maybe other domain could use the same approach in future developement and would use a different way of defining the device class.

[mboisson]

Since this pertains to sensors, wouldn't it better to handle de device class definition in the sensor case instead of in the general section? Maybe other domain could use the same approach in future developement and would use a different way of defining the device class.

This is done. I also added a string type value for sensors that do not report numbers but rather status.

[tomwpublic]

I added version information and made a couple of cosmetic edits. Please check for correctness @mboisson . Thank you for the contribution!