Parse battery status of HID++ 1.0 devices

This applies to K800 but it seems also to apply to M510. The numbers are based
on the HID++ 2.0 spec that state the following for GetBatteryCapability:

    If number of levels < 10 or if mileage is disabled then report are
    mapped to 4 levels this way.

    0%->10%     critical
    11%->30%    low
    31%->80%    good
    81%->100%   full

    i.e. to report battery low, FW send 25%, to report battery good, FW send 50%.
Merge pull request #23 from Lekensteyn/hid10-battery

Parse battery status of HID++ 1.0 devices