Replace permalink w/ reverse in get_absolute_url.

See
<https://docs.djangoproject.com/en/1.5/ref/models/instances/#the-permalink-decorator>.
Remove unused "include" from urls.
Abstract relation to User model. Fixes #1.

See
https://docs.djangoproject.com/en/1.5/topics/auth/customizing/#referencing-the-user-model.
Improve Django 1.5 compatibility.