Checking for a referral URI and if it is denied.

This patch implement a couple of additional checks.

1. That a referral URI is sent by the client.
2. Checks if there are any URI's configured that are not allowed referrers

Thanks to the heads up from Jamey Boje for forwarding this error report ..

strstr() [function.strstr]: Empty
delimiter. in /usr/share/drupal/modules/affiliates/affiliates.module
on line 1104.

Best, Paul