Prevent non-existing customers from creating orders

git-svn-id: https://plugins.svn.wordpress.org/paay/trunk@1012650 b8457f37-d9ea-0310-8a92-e5e31aec5664
PAAY button looks the same on 5 tested themes

git-svn-id: https://plugins.svn.wordpress.org/paay/trunk@1012651 b8457f37-d9ea-0310-8a92-e5e31aec5664