Google Music + Always Show Notifications

A couple things for you:

Firstly, I added the JS to scrape track info for Google Music.

I also broke the track notification call into it's own function to show notifications anytime you mess with the media keys. It's toggled via a a preferences check box for it. I had to fall back on a notification to update the property on AppDelegate once the user changed it (clears the listeners on close). Not super happy with that, but it does the job without any janky global dependencies or hitting up the NSUserDefaults directly.

I don't normally work in Objective C, but I tried to emulate the existing coding style where applicable. I welcome any feedback.