Tweak class Category. Add class CategoryForm.

Make the Category.tags model attribute non-required.

Create class CategoryForm, and write doctests for it. All doctests pass.
Write tests for class CategoryForm

Create class CategoryFormTestCase. Create several supporting functions in
`factories.py`.
Move hack from test_views.py to factories.py

The python 2 JSON encoder has a known issue with UTF-8 characters, and this
limitation is compensated for by creating sub-par random data. Move this hack
from test_views.py to factories.py. If we ever upgrade to python 3, we can now
remove the hack from a single place in the source code, instead of multiple
locations.
Fix errors in urls.py

Documentation for the /category/* URLs is wrong. Fix it. The
/category/create-form/ URL pattern is missing entirely. Add it.
Add tests for /category/* views.
Implement views for /category/* URLs.

Also add empty HTML templates. These templates are completely empty. However,
they must exist for the views to function correctly.
Update docstrings.
Allow HTTP GET /category/<id>/ to succeed.

Make it possible for a client to issue an HTTP GET request to /category/<id>/
URLs. In other words, a client can now fetch information about a particular
category. This change makes the category* views more in line with other families
of views, such as the item* and tag* views.

Update various views to point to issue HTTP redirects to this new view, and also
update tests to match.
Add HTML UI for /category/* pages.

UI is not yet done. In particular, `base.html` needs to be updated to show a
proper list of priority categories and their statuses. However, all other pages
are basically functional.
Make the `Category.name` model attribute required

Update the model itself, and update the form tests and factories to reflect that
change.
Add tools for using Category objects in templates.

Tools all expect a Category model object and return something based on that
object. Not all tools are working. Some work, others are simply stubs that
return 'FIXME'.

Ensure doctests in `category_tools.py` are run with rest of test suite.
Update README to list dependency
Fix bad wording in item-id.html template
Make UI present more info about categories