Changes to notification timing including critical bug fix with notifications.

Currently, they will only work until the app is closed due to incorrect boolean logic.
Also removed redundant calls to cancel alarm when changing value as the famework does this for us.
Added Debug line when alarm fires, helps to establish timings.
Added a call to get visible tweet on scroll.

This means that the notification will disappear when you scroll past the last notified tweet, rather than needing a extra refresh to clear it.
Merge pull request #89 from BlythMeister/Notifications

Bug Fixes To Notifications