Correct battery processing

This pull request contains two commits :
* The first one changes the returned value when "$LP_ENABLE_BATT != 1", in order to respect the API. The behavior was changed when the implementation of "_lp_battery" changed.
* The second one replaces the "pmset -g batt" parsing in Mac OS X. The current one breaks when there is no more battery because there is a warning line displayed before the searched value.