Subscriptions

The following new features:
* Table Listing of Payments for the admin to see charges
* Event handling of succeeded and failed charges
* Implementation of user account lockout when charges fail

Could not get stubbing of responses that StripeEvent makes of Stripe to work.

I think your name for `perform_subscription` could be better. It isn't really clear from the name what it does. Maybe something like `charge_customer_for_subscription` closer.