Made AtomEventObserver<T> an IObservable<T>,

although the implementation is still incorrect.
Made OnError a no-op.
Made OnCompleted a no-op.
Refactored AtomEventObserverTests

to prepare it to also be able to test the exact same functionality for the
OnNext method as the AppendAsync method contains when followed by a call
to Wait.
Implemented AtomEventObserver<T>.OnNext.