note about moving Heart and Stethoscope to a gem
note about message subclasses
notes about making boot more flexible

Users may not be using zmq, and may be using different implementations
of base actors.

Ultimately boot.rb should probably go away and be replaced by some kind
of dependency mechanism.
add actor_accessor to Configurable

Drys up the code to automatically dereference actor names
add and update specs for actor_accessor