notifications 6.x-4.0-beta6

Multiple bug fixes and minor improvements:
- Fixed: Duplicate emails when Content Notifications (used with Workflow module), #722432
- Fixed: WSOD on notifications_custom, refresh autoload after module enable, by yasheshb, #815812
- Do not send notifications to blocked users, #663186
- Updating subscription language after user account's default, #810736
- Added support for scheduled notifications, and notifications counter, #681786
- Fixed: notifications for newly created subscriptions sent right away whatever the interval is
- Group by language for PostgreSQL compatibility fix in query.inc, by halcyonCorsair, #811478
- Enabled "notify users of own posts" by default, #382482
- Added back (obsoleted) notifications_delete_subscription() so some old modules dont need upgrade.
- Extended Subscription objects with some methods for checking parameters, added notifications_check_subscription()