Add extra accessors for paypal notifications

This 3-commit long pull request includes:

* helper methods to test all status (like pending? or denied?)
* a helper method to access pending_reason
* a helper mehtod to access reason_code

All information can be found on [the same paypal doc page](https://developer.paypal.com/docs/classic/ipn/integration-guide/IPNandPDTVariables/).
You can see all status, all pending_reasons and reason_codes there.