* Added `AggregateRoot` interface.
* Changed internal Event values to avoid collision with external one -> Added `event_` prefix :)
* Refactor EventManager
- Deprecated `send` function, now use the `publish` function instead.
- Added `publish_list`.
* Refactored toy_example app (integration tests) using a `User` implementation of `AggregateRoot` to simplify the domain. :v: