Merge pull request #7 from pronamic/develop

Merge pull request #6 from pronamic/master
Merge branch 'develop'
Addition of update date in send notifcation and the column in the save method
Missing datetime format
whitespace
Addition of a check for auto_renew false.
Addition of sent_notifications set/get and call in the send_notification method
Inclusion of a sent_notifications param
Addition of since_last_reminder( DateInterval ) method.  Would also be beneficial to move this kind of check to the factory and make a setting.
Update to since_last_reminder method to make more optimal
Merge with develop
Reference to datetimezone for better comparisons
Merge branch 'develop' into feature-subscription-reminder