Split up ZephyrMessage.send_reply()

This will be convenient if somebody wants to mess with how reply zgrams are
built, who the recipients are, etc..
Refactor out into run_with_args
Rename --classes to --default-classes

--classes sounds like it subs you to the listed classes, whereas it actually
uses a hardcoded list of defaults.
Improve help messages
Allow subbing to extra classes on the command-line
Make main() a class method

This facilitates subclassing the Message classes to slightly change behavior.