Change battery display in powerarrow-darker

When AC plugged, the icon ac is used.
This theme used the icon ac when battery is absence. But I think it's not proper. 
People will be like to know they have connected AC when the battery is not fully charged.
So I changed it.
Whenever AC plugged, awesome should use the icon ac and show the charge.
If AC is unplugged, the battery must be present or the computer will not boot.
If AC is unplugged and the battery is present, awesome will use the icon battery and show the charge.
If AC is plugged without the battery, awesome will use the icon ac with the text "N/A"

I've modified lain too. Please check the pull request from lain.