setup.py: enforce django-polymorphic-tree 0.8.9 for Django 1.5 compatibility.
Provide hooks for structured admin monkey patching.

Projects can insert additional logic in the admin classes,
using the settings `FLUENT_PAGES_PARENT_ADMIN_MIXIN` and
`FLUENT_PAGES_CHILD_ADMIN_MIXIN`.

Template names are all implicitly loaded now,
so other classes can override the `change_form_template`.
The template can use `{% extends default_change_form_template %}` to
access the default logic.

Todo: documentation