Mock as_view when testing view class methods

Mock the ``as_view()`` method of a class-based view when
unit testing methods of a view class in order to properly
bind the view arguments and keyword arguments.

See http://tech.novapost.fr/django-unit-test-your-views-en.html
Merge branch 'develop' into 741-embed_widget
Update tests for resolve_list_uri()

Test all types of valid URLs for ``resolve_story_uri())`` as well
as an an invalid URL.

Update the return values of ``resolve_story_uri()`` in the case of
an unresolved URL.

Remove test stub for ``get_story_list()`` as I plan to rethink
how to test this functionality.

Addresses #741
Custom 404 template for widgets

We probably don't want the default 404 template showing up in
broken embed widgets, so use a custom template,
``storybase_story/widget_404.html`` for widget 404 errors.

This functionality is implemented via a mixin class,
``Custom404Mixin`` that short-circuits the default handling
of ``Http404`` exceptions.  This should be used sparingly.

Also moved ``VersionTemplateMixin`` to the
``storybase.views.generic`` module.

Addresses #741