Add deprecated WC_Paypal class

WC_Paypal was removed in 2.0 in favour of WC_Gateway_Paypal, this
patch deprecates the original WC_Paypal class & returns an instance
of the WC_Gateway_Paypal object instead of potentially causing a
fatal error.

classes/gateways/paypal/class-wc-gateway-paypal.php

@@ -750,3 +750,10 @@ class WC_Gateway_Paypal extends WC_Payment_Gateway {
 	}
 
 }
+
+class WC_Paypal extends WC_Gateway_Paypal {
+	public function __construct() {
+		_deprecated_function( 'WC_Paypal', '1.4', 'WC_Gateway_Paypal' );
+		parent::__construct();
+	}
+}