Real life handling of the ExpressCheckout

In ExpressCheckout mode there are no shipment costs and order items must be specified (even when doing ```do_express_checkout_payment()```). I made some changes to the order, ```$params``` building and gave more infromation in the example.

PS. I also think the address 'www.paypal.com/incontext?token=' at which user is redirected after calling ```$express_checkout->set_express_checkout()``` should also be set as a constant in Payment class.