Clean up debugging code and the need for mixins

Removed the ContentItemInlineMixin, this makes the code easier to
understand too. There is only a BaseContentItemInline now.
Set 0.9dev version number, drop Django 1.3 support
Simplify inline creation, drop Django 1.3 support

Django 1.4 has the fixes that the ExtensibleGenericInline uses; it
passes `kwargs` properly, and handles the excludes properly too.

The DynamicInlinesModelAdmin is removed too, as Django 1.4 has a
get_inline_instances() method, the PlaceholderEditorAdmin can use that.
All inlines can now assume they are created per request.