Settings update to allow different business and receiver e-mail addresses

The site I'm working on uses different e-mail addresses for the business (payment details going to PayPal) from the receiver e-mail (as used in the IPN notification).

So I added another setting to allow the two to be specified separately, and to check the IPN for the receiver e-mail, and not the business e-mail.

Working in SVN rather than CVS, so hope the attached diff is OK - it's a pretty simple change.