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".
Fix the changelog date of version 0.2
docs: remove tar file reference
Porting to Python 3.2

Removed compatibility_tools (because we aim to Python 2.7 and Python 3.2)
Performed 2to3 and modified
Usage of django.utils.six which adds dependency on Django >= 1.4.2
Port new code to Python 3 as well, fix six.with_metaclass() issues

Many thanks to @atheiste for the big porting steps!
Travis: don't test Django 1.4 against Python 3
Docs: mention python 3 support
Setup.py: update trove classifiers, read version from code.
Python 3.2 support confirmed.
Also update polymorphic version in the docs
Improve README example
Mention RTD link in documentation too.
README typoo
docs: mention Python 3 support in quickstart
Update Travis image in README
Update version numbers for 0.5 release
Remove Django 1.1 compatibility switch
Enable coveralls for testing