Empty aggregates, commands, and events.

This is a broad strokes first pass at some possible domain object. They
do not represent real domain driven design in any sense and are not
intended to be used as the basis for a live application. The domains are
not mapped, the contexts not bound. The language is not ubiquitous and
no domain experts have been consulted. This is a proof of concept for a
possible CRM application using event sourcing. It is a demo for
educational purposes only.