Better abstract models

Biggest component added here is new abstract models. Previously the concrete Organization model and friends included helpful methods for working with group accounts that the base (abstract) models lacked. A new set of abstract models include these methods to add this functionality to classes using the base models. Thanks to @nemesisdesign for suggesting, spearheading, and implementing this - 🥇

A few documentation and minor code updates as well courtesy of @amadornimbis, @bashu, and @Robertchristopher - 🎁

Last but not least, while not in Travis yet, the project tox configuration now covers Python 3.6 and Django 1.11 (in progress).