Add BusyBird::Watcher interface class. Tests and POD for Timeline and Main are revised so that their watch_* mathods should return BusyBird::Watcher objects.
t/Timeline.t, t/Main.t: add tests that the watcher inside callback is the same as the watcher outside.
Add BusyBird::Watcher::Aggregator, which is just a mix of BusyBird::Watcher and Async::Selector::Aggregator. Revise Timeline to use BusyBird::Watcher::Aggregator, thus conforming to the spec.
Watcher: set version.
BusyBird::StatusStorage: update POD. it now states it's an interface class and an implementation must be its subclass. add tests to check that.
StatusStorage::Memory and Test::StatusStorage::AEDelayed now inherit StatusStorage.
Minor updates in PODs.
StatusStorage: remove new() method from the spec. Constructors do not have to be new(). It's up to the implementation.
t/Main.t: add tests for cyclic refs.
Implement Main. Pass the test except for todo.