Sending item details

As per the [CyberSource API docs](http://apps.cybersource.com/library/documentation/dev_guides/CC_Svcs_SO_API/Credit_Cards_SO_API.pdf) there are several additional fields that items accept. I made changes to send these along too.

An example:

```php
$cybersourceApi->add_item(
	'55.00', // unit price
	1, // quantity
	array(
		'productCode' 	=> 'electronic_good',
		'productName' 	=> '50 credits',
		'productSKU' 	=> '111',
	)
);
```