Simplify events
Centralize EventStore integration
Rename EventSourcedAggregateRoot to AggregateRoot
Remove DomainEvent namespace
Rename LifeCycleEvents to InternalEventSystem
Move repository to main namespace
Rename EventSourcingRepository to Repository
Clean up and simplify the lib

- Remove internal event system and stick with conventions
- Remove repository
- Remove exceptions
- Adjust AggregateRoot and decorator so that reflection is
  no longer needed
- Simplify AggregateChangedEvent, allow to set version, but
  only once
Add tests and quickstart