Improve admin form of OEmbed item, make more compact
Fix unnecessary query for parent in Placeholder.get_absolute_url()
Improve DoesNotExist message in PlaceholderFieldDescriptor.__get__
Fix field required indicator for "max size" field of oembeditem
Add first admin test for PlaceholderField.

This allows testing Placeholder and ContentItem inlines.
Test placeholder assignment during admin POST request.
Fix Django 1.5 compatibility.

get_inline_instances receives an additional parameter now.