Uninstall extension after 120 days

- Added an alarm created during setup to uninstall the extension after 120 days.
- Added a listener for that alarm.
- Renamed the alarms to differentiate between the notification timeout alarm and the uninstall alarm.
- Added a check to clearNotifications to only run the body if it's the notification timeout alarm.
- Moved the onAlarm.addListener for the notification timeout next to clearNotification, where it's a little clearer.

This fixes issue #40 
nit: don't need { } here for one-line if-statement