Pinax-referrals

* [Release Notes](https://github.com/pinax/pinax-referrals#401)

* [Release Notes](https://github.com/pinax/pinax-referrals#400)

* [made user blankable](https://github.com/pinax/pinax-referrals/issues/38)

* Use Django 2.0 import for `reverse`

* Change `is_authenticated` to property in models.py

* Add Django 2.0 support
* Drop Django 1.8, 1.9, 1.10 and Python 3.3 support
* Convert CI and coverage to CircleCi and CodeCov
* Add URL namespacing
* Update installation requirements for django>=1.11
* Add PyPi-compatible long description
* Standardize documentation layout
* Remove unused doc build support