Pinax-ratings

* [Release Notes](https://github.com/pinax/pinax-ratings400)

* Improve test clarity and coverage
* Improve documentation
* Fix OverallRating manager method

* Add templatetag tests, model tests

* Fix import reverse

* Add Django 2.0 compatibility testing
* Drop Django 1.8, 1.9, 1.10, and Python 3.3 support
* Add URL namespacing (BI: urlname "pinax_ratings_rate" is now "pinax_ratings:rate")
* Move documentation into README and standardize layout
* Convert CI and coverage to CircleCi and CodeCov
* Add PyPi-compatible long description