don't transfer tips to participants who don't want to receive them
don't transfer tips to participants who don't want to receive them

fixes #916

eXcomm/gratipay.com,mccolgst/www.gittip.com,studio666/gratipay.com,eXcomm/gratipay.com,studio666/gratipay.com,gratipay/gratipay.com,gratipay/gratipay.com,studio666/gratipay.com,studio666/gratipay.com,gratipay/gratipay.com,mccolgst/www.gittip.com,eXcomm/gratipay.com,mccolgst/www.gittip.com,eXcomm/gratipay.com,gratipay/gratipay.com,mccolgst/www.gittip.com