Support JSON cassettes by passing a filename ending in .json

For example, for JSON do:

```php
	/**
	 * @vcr AccountInvoiceRequests.json
	 */
	public function testInvoices()
	{
	}
```

And YAML:

```php
	/**
	 * @vcr AccountInvoiceRequests.yaml
	 */
	public function testInvoices()
	{
	}
```