Improve notification handling

* Adds a button to the notification
* Since background.js is an event page, use chrome.alarms instead of SetTimeout (see Issue #20)
* Since background.js is an event page, use chrome.notifications instead of HTML5 notifications (see Issue #20) -- except on Linux, which doesn't have chrome.notifications support
* Start adding filtering for different types of events
* Bonus: some style fixes, like using ' instead of "
Odd, because chrome.notifications.create seems to run successfully on Linux. The callback runs, but I don't see any UI indication. You might want to check this with the extensions team.