Add line items to information sent to Authorize.net

It would be great to add individual line items to the information sent to Authorize.net.

The attached patch does this - with a couple caveats.

When sending the line items as "x_line_item", the information is only recorded in Authorize.net and sent via the customer email. In order to have the line item display on the administrator Auto-Receipt, they also needed to be added as merchant defined fields.

The other caveat is that the attached patch only works with AIM - not the card on file transactions.