New Spoil and Ping message types.
Integrate Spoilable with publisher.
Refactor to match "Deprecating the Observer Pattern".
Introduce publisher type.
Publisher should not depend on Message type.
New Changeable trait.
Changeable class only accepts Change messages.
New EventSink trait.
Visibility improvements.
New TestSimpleLiveValue and Unspoilable.
Fix spoils type.
Remove debugging.
Test subscribe.
Make subscriptions disposable.
Test subscription disposal.