Added battery plugin

[shahlin]

Provides information about battery such as the percentage, time remaining before empty, vendor, technology and energy consumption.

[pnhofmann]

Thanks!

upower is something linux specific or is there a mac version available? Which of course would be no problem - not every plugin needs to support every platform.
Howerever does mean, we should only enable this feature if we are on a linux-machine with upower installed. Since how to do that is not well documented (work in progess), I have added 2-line commit to do so. Hope you are OK.

I just don't know: Does battery always have this path: '/org/freedesktop/UPower/devices/battery_BAT0'? On my FreeBSD machine it's available as '/org/freedesktop/UPower/devices/battery_devxbatteryx0' - I'm honestly not the freedesktop-expert.

I'll test this plugin on my linux system, investigate if there is another way than hard-coded '/org/freedesktop/UPower/devices/battery_BAT0' and merge afterwards.

[shahlin]

Yes, my bad, I should have made it Linux platform only. I tried it with Mac and it doesn't work.
I tried testing on other Linux distros and it seemed to work on all for now. Here are the screenshots:

• Ubuntu
• Debian
• Fedora
• Manjaro
• Antergos

[pnhofmann]

Finally had time to test... Works really nice! Interessting, FreeBSD called battery_BAT0 different - thanks for testing different different Linux-Distributions; looks really like on Linux it's allways BAT0.

Actually really like this one! Thanks!