Fixed #12508 - Added ability to dynamically add inlines in the admin app.

Refs #13. Also introduces an ``empty_form`` attribute on formsets to make it easier to implement dynamic forms.

Many thanks to Zain Memon for the initial patch from his Summer of Code 2009 project, Stanislaus Madueke for his django-dynamic-formset app and all the other people helping out.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12297 bcc190cf-cafb-0310-a4f2-bffc1f526a37