Update battery plugin to show calculating - OSX

Instantly after removing the charger, I have found that the "AvgTimeToEmpty" value can swing to rediculous values. Apple's current claim is that the max battery life is 12 hours, so any value larger than this is considered a "time is being calculated" value.
Merge pull request #2539 from chriswiggins/master

Update battery plugin to show calculating - OSX