Finish referral functionality, improve PayPal detection

I'm new here to Drupal development, and this is my first shot at a patch, so let me know if I make any faux pas here.

The main fix of this patch is to implement the referral functionality as discussed at the end of this thread: http://www.ubercart.org/forum/development/896/userpoints_module

This code checks to see if the referral module is installed, sees if there is a referrer, and respects the setting of USERPOINTS_AWARD_TO. [note: if you want the module to give points to the purchaser only by default (like it does now), you'll want to change the default setting of USERPOINTS_AWARD_TO to 1, not 0] It goes through the process of awarding points once for the purchaser, and then repeats it for the referrer.

I rolled two other minor changes into this as well. First, I made it so USERPOINTS_UC_EARN isn't automatically casted to int -- I originally wanted my store to give 0.1 points/$ spent, and have each point be redeemable for $1 in goods. It took me quite a while to figure out why that wasn't working. I don't see a reason why that number should have to be an integer, so I stopped casting it that way.

Second, this module didn't work at all for me at first. I believe it was the if statement that had this condition strtolower(substr(request_uri(),1,22)) == 'uc_paypal/wps/complete'. My site is located in /drupal-5.5/, so that would presumably never work. As a result, I changed it to look for 'uc_paypal/wps/complete' anywhere in the string, and I also added 'uc_paypal/ipn/' since PayPal seems to hit that automatically after a purchase (this would avoid problems if the person didn't return to the website after payment).

Caveat: I really don't know much about the Ubercart/PayPal interaction, so this might be wrong. It seems to work for me, and I was actually surprised at first when it didn't award points twice (once for hitting the ipn url, and once for the wps one). The 'uc_paypal/ipn/' part might be wrong, unnecessary, or misguided. If someone has a better idea about how this should be done, please let me know (actually, I just tested it, and points aren't assigned via the IPN hit at all, so I guess that should be removed). FYI, my IPN URL as set in the store's account in the PayPal Sandbox is 'http://<mysite>/drupal-5.5/uc_paypal/wps/complete'. With all that in mind, searching for the string in request_uri() is likely a better approach than expecting it to be in a specific place. An even better approach might be to determine the actual URI it should be using, but I'm not sure it would add much.