Fixed #849 -- Improved login_required view decorator to save query-string...

Fixed #849 -- Improved login_required view decorator to save query-string parameters. Also added documentation on the django.contrib.auth.views.login view to docs/authentication.txt

git-svn-id: http://code.djangoproject.com/svn/django/trunk@2954 bcc190cf-cafb-0310-a4f2-bffc1f526a37