Add time since unplugged feature

Basic idea is to add one more menuitem in advanced mode that counts how much time you've actually spent on battery since you've unplugged power source.

It is very similar to Android's "Time on battery" or "Time since last plugged".

Implementation is a basic timer and counter that counts 15 seconds intervals. When mode changes from AC to Battery timer starts and shows "Time since unplugged: 00:01". When mode changes from Battery to AC timer stops but value is still stored for information purposes in shown in menu as "On battery while last unplugged: 00:12".

This is my first time I've opened Xcode and write Cocoa code, please review :-)

Thanks.