Added SalePrice accessor

Amazon sometimes has special offers which they hide in
Offers/Offer/OfferListing/SalePrice/Amount rather than
Offers/Offer/OfferListing/Price/Amount.

OurPrice then lists the "regular" price, while you probably
want the sale price. So to get the current price, use something
like:

   my $price = $prop->SalePrice() ? $prop->SalePrice() : $prop->OurPrice();

Maybe OurPrice should even be patched to display the SalePrice
when it exists and the Price value when it doesn't, but I'll
leave that up to the maintainer of the module.