Allow AC only state
renamed option list-batteries into list-power-supplies to show battery and AC
don't fail if there is no battery, display icon for AC power supply
fail if there is no battery with the provided (optional) battery suffix
remove trailing whitespace
Allow execution of command when left clicking on the tray icon
Reviewed and fixed list of options in README
Improve Makefile comment
Allow building against gtk+-3.0
Compile for c99 standard
Fix compiler warning with -pedantic