Add support for ActionsObservable.of(...actions) etc

[jayphelps]

This would make it far easier to create them for tests and such:

const action$ = ActionsObservable.of({ type: FIRST }, { type: SECOND });

// instead of the current:

const input$ = Observable.of({ type: FIRST }, { type: SECOND });
const action$ = new ActionsObservable(input$);

I was waiting on ReactiveX/rxjs#1876 but it might be a while for that is resolved so we probably should provide our implementation in the mean time.