Added more methods for subscription specific IPNs 
Added more methods for subscription specific IPNs 

Added two more methods for subscription specific IPN `is_subscription_payment` and `is_subscription_failed` as described in PayPal docs: 
https://www.paypal.com/us/cgi-bin/webscr?cmd=p/acc/ipn-subscriptions-outside
GamesDoneQuick/django-paypal,rsalmaso/django-paypal,rsalmaso/django-paypal,spookylukey/django-paypal,GamesDoneQuick/django-paypal,spookylukey/django-paypal,spookylukey/django-paypal,rsalmaso/django-paypal