notifications 6.x-2.0-beta6

Bug fixes and polishing. Changes since beta5:
- Removed logging functions, now using messaging logging facility.
- Added content type subscription update when content type name changes, #429296
- Fixed wrong db_placehoder type for node autocomplete callback
- Fixed inconsistent fieldset for 'Skip notifications' in node and comment form.
- Added comment about Immediate notifications not working with AJAX forms, #444010
- Fixed wrong names for sending methods on subscriptions form, #443802