Article series now stored in the separated model, which uses django-treebeard. Also, ArticleCreateUpdateForm introduced with some stolen methods, that makes tree-like drop-down options available in the admin panel.
TODO: select2 or other autocompleting widget for category selection.
South migration does not support field renaming from CharField to ForeignKey so two different migrations generated. Think twice on model design!
Merge branch 'feature/extend_atricle_model' into projects/publishing

Conflicts:
	periodicals/models.py