Changed related_name in foreign key "created_by", reformatted code, readme and license

I had to change a `related_name` in the `Item` model in order to avoid a conflict with [django-media-tree](https://github.com/samluescher/django-media-tree/blob/master/media_tree/models.py#L319).

The rest is merely making the code more beautiful, readable and maintainable (PEP 8).
README is now in reStructuredText since this is the de-facto standard in Python projects.

Additional goody: There is now a `django-todo` package [on PyPI](https://pypi.python.org/pypi/django-todo). This is how I created it for version 1.3 prior to the changes:

    ./setup.py register
    ./setup.py sdist upload

I've updated the Wiki pages to reflect the latter fact (stable release installation). Let me know your PyPI user name to make you package maintainer.