Refactor JavaScript dialogs API

The phantom’s approach to handling javascript dialogs is convenient and familiar: instead of firing Alert, Prompt and Confirm events, we should use onAlert, onPrompt and onConfirm callbacks. It would be also convenient to have a complimentary Dialog event which dispatches dialog class.

[ebidel]

I like it.

[dgozman]

I think onFoo is a nice way to subscribe to events. We use it in our testing harnesses a lot.

page.onAlert(message => {
  console.log(message);
});