Handle stale ContentType models (no longer referring to a model)

This builds on top of a fix in Django 1.6, and has a workaround for
Django 1.4 and 1.5. When the base class points to a model that no longer
exists, it will be silently dropped in the polymorphic queryset results.

This behavior is identical to iterating over results when the derived
table doesn't have the object anymore.
Port documentation to Sphinx, cleanup README
Mention the word "inherited models" in the docs more often

..to give some SEO juice to the project because developers are looking
for "inherited models" instead of "polymorphism".