Aggregate Root Test Scenario

The current `EventSourcing/Testing/Scenario` seems to have fallen behind. This pull request added some functionality such that you can easily test your `EventSourcedAggregateRoot` objects without using a command bus.

Nb: I also added an example which was basically copied from the `event-sourced-domain-with-test` but changed such that it uses the new `AggregateRootScenarioTestCase` class.
It might be better to use a different example and also add a testcase for `EventSourcedEntity`.
Can you add this test to the already existing example? You can rename the files there (this will be `InvitationTest.php` the existing test `InvitationCommandHandlerTest.php`?).