Allow American Express cards to validate
Re-added handling for non-standard result message with error code 4
Merge pull request #581 from ryscher/paypal-amex-fix

Re-added handling for non-standard result message with error code 4
Merge pull request #571 from datadryad/paypal-amex-fix

Allow American Express cards to validate
Merge remote-tracking branch 'upstream/dryad-master' into dryad-master