Casper#waitForAlert()

This one introduces `Casper#waitForAlert()` to wait until an [alert](https://developer.mozilla.org/en-US/docs/Web/API/Window.alert) is triggered:

```js
casper.waitForAlert(function(response) {
    this.echo("Alert received: " + response.data);
});
```

Waiting for Travis to confirm all is fine.