Pending reason and checkout status

Hi,

I add the pending reason to the payment pending exception, for better handling the reason in controller.

Additional I fixed the wrong Checkout-Status. The API-Doc says:

It is one of the following values:

    PaymentActionNotInitiated
    PaymentActionFailed
    PaymentActionInProgress
    PaymentActionCompleted

In https://developer.paypal.com/webapps/developer/docs/classic/api/merchant/GetExpressCheckoutDetails_API_Operation_NVP/

I tried and can confirm.

Would be very kind of you to merge this and also create new tagged version.

Thanks

Sven