refactoring frontlight device to more generic power device
cache battery level in BasePowerD
Since we need to pull it frequently but usually it doesn't change
that often.
show battery icon in crereader page header
fix isCharging name conflict
add battery status pulling functions in KoboPowerD
Status files found at
http://www.mobileread.com/forums/showpost.php?p=1750642&postcount=9.
Merge pull request #433 from chrox/master

show battery icon in crereader page header