Listen for backgrounding and foregrounding events in Timer

[hawkrives]

And fire the timer if it's time has come while we were in the background.

I think setTimeout doesn't fire while in the background, but we should be able to store the current time when we background/foreground by listening to the AppState module, and then calculate if the timer should have fired while we were backgrounded.

(I noticed this while watching the Bus view and switching between it and my ebook reader.)