Fixed #12815 -- Added TemplateResponse, a lazy-evaluated Response class. Thanks to Simon Willison for the original idea, and to Mikhail Korobov and Ivan Sagalaev for their assistance, including the draft patch from Mikhail.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14850 bcc190cf-cafb-0310-a4f2-bffc1f526a37