Allow specification of product field that contains productCode for tax calculation

Here's a first stab at this.

I changed a couple of invalid line item properties that we're invalid along with the addition for product code in a custom field:

-      $item->productName = $line_item_wrapper->description->value();
-      $item->productSKU = $line_item_wrapper->label->value();
+      $item->productName = $line_item_wrapper->line_item_label->value();
+      $item->productSKU = $line_item_wrapper->type->value();

Removed a redundant comment.

Another patch, this one removes the line that assigns and ID to the item, since cybersource only uses the index, not ID, and it only indexes from 0. (If item 0 is missing, it rejects, and returns an missing field error).

$item->id = $line_item_wrapper->line_item_id->value();

I don't know much about how entity_metadata_wrapper works, but I hope we can trust the Iterator and ArrayAccess to consistently sort the items, otherwise we'll have to flatten it out into a separate array and sort it before preparing the order, and remapping the response to line items will be a bit of a pain.

I take that back, the missing id field was causing CyberSource to return a SOAPException about java.lang.NullPointer error or something like that.

This just uses the delta since it's going to get reassigned anyway.