Fixes for circular dependencies preventing Entity destruction, and Listener re/un-assignment (#129)

* Don't rely on the _entities dictionary in the Entity finalizer.

In the presence of circular references the object being finalized may
well have already been removed from the WeakValueDictionary that makes
up the _entities store. See: https://gist.github.com/willstott101/8544a7c966b0788ff4f93da410e9fea7

* Support un-assigning listeners in Entity, and fix bug with reassignment.

* Add test for listener reassignment corner-cases