[Mac] Add setAlternateImage support in NSStatusItem

Adds support for alternate (active) tray icon for Mac (setAlternateImage in NSStatusItem).
Fixes https://github.com/rogerwang/node-webkit/issues/442 and https://github.com/rogerwang/node-webkit/issues/366

Usage: var tray = new gui.Tray({ icon: 'icon.png', alticon: 'alticon.png' });

Please note: I've only tested this on a Mac, where it works fine. Someone please check the code and make sure it works elsewhere too.