Use "product feature" to determine if a node is a gift cert

Hey Jason,

Thanks for the work on this. A few thoughts.

- In uc_gift_certificate_create_new_certs() we're looking for either a msg_to_recipient or email_address attribute, but this isn't flexible enough to allow us to create printed certificates. I would suggest that instead of this, we should just look for the "Dollar Amount" attribute first (and then we can check to see if it's an emailed certificate or not). Or, alternatively, we can use that as a third elseif, as I've done here:

// Check if the product is a gift certificate
      if ($data && $data['attributes']) {
        // Attribute is stored using label
        foreach ($data['attributes'] as $label => $vals) {
          $attrib = _uc_gc_find_attrib_by_label($label, $product->nid);
          
          if ($attrib->aid == $email_attrib->aid) {
            $is_gift_cert = TRUE;
            $recipient_email = $vals[0];
          }
          elseif ($attrib->aid == $msg_attrib->aid) {
            $is_gift_cert = TRUE;
            $msg_to_recipient = $vals[0];
          }
          elseif ($label == 'Dollar Amount') {
            $is_gift_cert = TRUE;
            // Use the purchaser's email?
          }
        }

- Since we won't have an email address to notify the person (recipient) with, perhaps we should attach the purchaser's email address, and send them an email with the code and a notification that a certificate was generated and will be shipped to them. This could be optional, of course.

Anyway, I feel like Dollar Amount will always be a part of a certificate, printed or electronic, so we should probably include that in the logic somewhere.

- You'll also notice I added $is_gift_cert = TRUE to the 2nd elseif. This was missing but I'm not sure why; if the product has a msg to recipient, that probably means it's a certificate, but I'm not sure this will always be the case. I am wondering if not having it there would cause some certificate purchases to fail.

I had to make these changes to my working version of the module to accommodate for printed certificates, which are always huge around the holidays. I'd like to rethink some of this but without negating the attribute-checking logic you've written. I think the logic you've included is good, to determine whether or not it needs to use the $vars[0] as an email address or a message, but we can improve it a little bit for the use-cases where a recipient might be receiving a gift card in lieu of an email.