Make ``Formset.__getitem__`` O(1), rather than O(n).  If you override ``__iter__`` you now need to also override ``__getitem__`` for consistant behavior.  Thanks to Carl and Russ for the review.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16770 bcc190cf-cafb-0310-a4f2-bffc1f526a37