Show alert for critical battery

[yatharth]

This PR addresses #103:

Feature Request: Could there be an option to have a pop-up or a dialog box when your battery drops below a certain level?

I wanted to feature enough to hack it together myself right then. ‘Hack’ as I’ve never written Objective C code before; I worked off of this PR.

The alert doesn’t steal keyboard focus. When it is active, the menu bar item can be activated but not interacted with.

[yatharth]

Re localization: I have wrapped my strings in NSLocalizedString (and provided appropriate context), but I have not done anything else. I don’t know if I should have (but maybe I could try if given a pointer?).