Add deprecated WC_Paypal class

`WC_Paypal` was removed in WC 2.0 in favour of the new `WC_Gateway_Paypal`, this patch reintroduces the original `WC_Paypal` class as a deprecated class which just returns an instance of the new `WC_Gateway_Paypal` object (instead of causing a fatal error).

Another patch like #2430 & #2431 aimed at making WC 2.0 more backward compatible.