Payment History Importer does not support variable prices

[wvega]

When the Payment History Importer is creating a new payment, it does not include information about variable prices, even if the associated Download supports them.

I think the importer should try to match the payment's price with the available prices and include relevant information when possible.

Using EDD_Manual_Purchases::create_payment() as a reference, I was able to modify the plugin to do what I just described, but those modifications were made on top of the modifications included in #20. I'll wait until we decide what to do with that pull request, before I submit another one, to avoid unnecessary merge conflicts.