Made CSRF middleware skip post-processing for 'csrf_exempt' decorated views.

This commit also decomposes the decorator into two decorators which can be
used separately, adds some tests, updates docs and fixes some code comments.



git-svn-id: http://code.djangoproject.com/svn/django/trunk@9815 bcc190cf-cafb-0310-a4f2-bffc1f526a37