Itemized payment data for Paypal WPP

Correct me if I'm wrong, but according to paypal's NVP documentation it looks like adding this feature would be as simple as injecting item data into the DoDirectPayment NVP POST. I will start working on a proof of concept patch based on this understanding.

Any specific features that should be included for this patch to be considered for inclusion?

OK, WPS and WPP formats are different enough that I don't think they could be combined easily into a single function.

Here's a proof of concept patch that i've tested very briefly.

writing this patch raised 3 minor questions:
1. One of the optional fields for each item in the itemized receipt is the item's taxes. But, uc_taxes doesn't appear to offer a quick, clean way of retrieving the taxes for a single item -- only the entire order. So, for now, itemized taxes are omitted. (The function uc_taxes_apply_item_tax() requires an $item arg as well as a $tax arg, which means that any calling function is responsible for marshalling all the applicable taxes... this is out of hand. Ideally uc_taxes should offer a function to either generate taxes for the order and return an itemized listing of the taxes for each item, or a function to retrieve all the taxes for a single item without any args besides the $item itself. (I know this should be its own issue. Just documenting how it applies here.))
2. Another optional field, description. What should go here? implode()d attributes?
3. Should enabling itemization be configurable? I can't think of plausible use case in which someone would NOT want an itemized record of payment in paypal.

This patch is working ok for me.
I still haven't figured out how to format a patch to appease testbot =[

Here's an update the adds any line items to the itemized total.
Untested.