Cleanup, example and overall improvements to django-threadedcomments

Hi,

I've been looking into getting django-threadedcomments up to date, with clean examples. I hope you accept this first series of changes. It contains:

- An example app
- Fixes when deleting parent comments
- A `render_comment_list` tag and `get_comment_count` so that `{% load comments %}` can be replaced by `{% load threadedcomments %}`
- South migrations
- ...and overall cleanups of the code.

These were all parts I missed in the application, and I hope it saves other peoples time too.
Thanks for your time to include these changes.

Diederik