Simple request: add modules to respond to paypal ipn for recurring payments

uc_recurring_hosted handles IPNs for paypal recurring payments - basically messages from paypal that notify drupal what happened when a recurring payment was charged, cancelled, suspended, etc.

the function uc_recurring_hosted_paypal_ipn() does the legwork of making the order on the comment and logging any payments, but there's no opportunity for contrib to respond to such events.

This is a rudimentary patch that would allow contrib modules to respond to IPNs.
Yes there are existing ubercart hooks that get called, for example, when an IPN gets posted "Completed" and uc_payment_enter() gets invoked, but not all circumstances trigger such hooks.

This patch adds a hook, uc_recurring_hosted_ipn, that gets called at the end of this callback function.
Maybe not this patch exactly, but something needs to give contrib the opportunity to respond here.