Productlist & Hash support

Class changed to support both the productlist and hash key method.

The productlist supports only 1 product! In the example there is added
1 product (PROD1). Add this product to the ideal test dashboard or add
a product of your own but don’t forget to change the itemNumber in the
demo!

Also, a file testpayments.php is added. This is an easier way to help
you do the 6 test payments.

Besides the two above: a little mistake in ideal.class.php was fixed
which made it impossible to add AlphaNumeric itemNumbers which is
allowed within the iDeal platform.