Self registering handlers

This PR represents one approach I drafted for "self-registering handlers". The implementation uses metaclasses to cause all *imported* subclasses of BaseHandler to be registered and present.

While I'm happy with the power and elegance of this approach, as exemplified by the number of lines of code the changeset removes, it also has some implicit behaviors that could lead to elusive bugs.

I humbly send this pull request for your consideration. I look forward to your feedback, but please feel free to accept it and change it, reject it and use it for inspiration, or reject it and disregard.