Add PAYMENTACTION Selector

[angelleye]

Right now everything gets processed as Sale for all of our PayPal integration. Need to add an option in settings so that people can set Sale or Authorization, and then use this in the requests.

For now, if they set Authorization they'll just have to capture the payment from within their PayPal account. #36 will expand on this, though, to more tightly integrate the whole flow into WooCommerce.