* Add hwmon support (mainly known from lm-sensors)
This commit adds initial support for linux hardware sensors, exported
through sysfs.
Details of the interface can be found at
https://www.kernel.org/doc/Documentation/hwmon/sysfs-interface
* Add end-to-end test with some real life data
* Cleanup comments on hwmon collector
* Drop raw sensor name from hwmon output
* Let the sensor label be "sensor"
* Add hwmon short description to README.
