Self registering handlers

I started merging jsonpickle with my jaraco.modb project and I realized that the self-registering-handlers functionality, while convenient for registering handlers when they're created, doesn't facilitate using an existing handler for another class. This addition creates a class method on the BaseHandler (and thus derived subclasses) to register a handler for types created after the handler class. This new class method actually simplifies the implementation a bit as it creates a single point where handled types are registered.

This request additionally adds documentation describing the usage and updates the documentation in the api docs to suppress discussion about the 'registry' which is now an internal implementation detail.