Changed terminology: callbacks are now functions
remove the public post_message method: moving to a more restricted interface
moved over entirely to a controlled message posting interface -- can no longer post arbitrary messages through the public interface -- removed some unit tests that were no longer relevant
removed obsolete method and comments
fixed add_function so that it now only adds a single function, rather than a list to be called in sequence.  added add_callbacks, which stores a responder, and injects a function in the worker which will pass a message out to the responder