Remove notification level after hiding. Fixes #200

    Not sure if hide() could ever be triggered without triggering show() first,
    but set the level default inside hide() just in case.
Fix bad enum value on Annotator.Notification
Make notification tests fail right
Merge branch 'issue_200_notifications_cleanup' of https://github.com/6a68/annotator into 270-notification-level-badness

Fixes #270