Added method to create EventResponse object by JSON string

WebPay's webhook POSTs just an JSON string data, so I want to create `EventResponse` object from JSON string.

For example, receive the webhook request by following script:

```
<?php

use WebPay\Event;

$jsonString = file_get_contents('php://input');

$event = Event:: createFromJsonString($jsonString);
echo $event->id; // => evt_bWrgT9XXXXXXXX
```