The module now creates a widget for each battery device it detects and
shows the status for each of them (I don't know of anyone with more than
a single battery, but if I'm overhauling the module anyhow, might as
well do it comprehensively).
fixes#117
Seems like subprocess and friends (Popen, communicate) are not so easy
to mock cleanly. Therefore, start from scratch and carefully write test
by test, until (at least) the old test coverage has been restored.
