Discontinue use of GET.

[patricknelson]

Hi, first off: I've been using this for a few years, so thanks for building this :+1: It looks like PayPal has issued a statement that, for security reasons, they will be discontinuing the use of GET requests across the board for NVP: https://www.paypal-knowledge.com/infocenter/index?page=content&widgetview=true&id=FAQ1927&viewlocale=en_US

That being said I think the change should be fairly simple: Move to POST and migrate the query data in ->send_api_call() from the query string over to the HTTP POST body using cURL. This shouldn't cause any issues since cURL is already required by this class anyway.

Since I may have to do this soon, I'll likely issue a pull request within the coming month or so.