Prepared for SP 4-6.

- Main and lesson list pages overhaul.
- Lessons now inform about internet requirements and publisher.
- PDF support for <obraz> (by sorl.thumnail with ImageMagick)
- Add summary and picture for Sections, description for Lesson.
- Level slug and meta dc:audience value can be different.
- Per-level lesson packages instead of global ones.
- Lesson packages are now rebuilt after import.
- Add option for allowing incomplete sections on import.
- Django 1.6, `django-piwik` replaces `piwik`.