PayPal EC Updates

* Remove osC_Action=close page; move html redirect code to function and call the function when a redirect is needed (one less redirect, yay).
* If EC is selected as a payment method and the shipping address is invalid, skip the checkout payment page and redirect to the checkout confirmation page when a valid shipping address has been entered.
* Also verify transaction in before_process()
* Add useraction=commit so the button label in EC is "Pay Now"; process the order immediately when returning back to the store and skip the checkout confirmation page.