Fixes and Django standard adoption

This release includes mainly bugfixes in the Admin interface (see PRs #106, #109, credits to @dunkelstern) and in the behaviour when working with Many-to-Many-relationships to self (see PR #112, credits to @brki).
You'll also notice a change that allows usage of Versionable objects that is closer to the Django default behaviour, by using objects as filter lookup values (see PR #113 for details, credits to @brki).