Replace 'Index' View with Generic ListView

As explained in the django documentation, example with Publishers,
Authors and Books.
Hook Generic ListView for Categories to Urls
Add Generic ListView for ProductCategories

And correct a whole lot of stupid mistake due to ignorance and fatigue.
Add Products to List of Categories as Sub List

Needs to be linked yet. Error in if condition, 'empty' categories need
to be pointed out (see product_categories_list.html template).
Remove Ineffective If Statement

Right now there's no condition that checks, whether there is a product
for a given category at all. This is most irksome.
Remove Obsolete Views

Removed the original index view (replaced by a ListView).
As there is no more need for a separate column for a product list (which
moved into sublists of the category list), the product view has been
commented out.
Remove URL for Products

There won't be a separate page for product lists, so there's no need for
a extra url, view etc.
Remove Links from Category List

Remove the link that used to point to the now obsolete product lists
page.
Eventually there will be need for a link, in order to trigger the
accordeon.
Write HTML for Sublist
Add Meta Class for Custom Ordering of Objects

Add Meta Class to Models as described in https://docs.djangoproject.com/en/1.6/topics/class-based-views/generic-display/
Add Condition for Empty List

As described in
https://docs.djangoproject.com/en/1.6/ref/class-based-views/generic-display/
there's an extra template tag "empty" for this :-)
Update README

Split original Wishlist into Done, Todo and Obsolete. Add some details.
change title "documentation" to "Installation"
Add item to TO DO List
Add Field for Page Order to Admin Interface

Trivial.
Change Order in Meta Class

Change from descending to ascending. Might prove to be foolish.
Add pager-order to ListView of Page Admin

As stated in the title. Admin page could be more streamlined but that's
probably not part of the contract.
Update To Do List

Add some items and comments concerning models.py.
Remove obsolete comments

Some comments were obsolete. Others needed to be moved to the To Do list
in the README
Correct Markdown Syntax for Nested Lists

Right.