Fixed #8630: finished the custom comment app API that was left out of 1.0....

Fixed #8630: finished the custom comment app API that was left out of 1.0. This means it's now possible to override any of the models, forms, or views used by the comment app; see the new custom comment app docs for details and an example. Thanks to Thejaswi Puthraya for the original patch, and to carljm for docs and tests.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9890 bcc190cf-cafb-0310-a4f2-bffc1f526a37