Fixed check for merchantSignature

Fixed the broken check for merchantSignature. It always returned the
calculated signature instead (as advertised by method signature) the
evaluation if the submited merchantSignature is valid.
Merge pull request #2 from ICANS/fix-verify-notification-signature

Fixed check for merchantSignature
Make paymentMethods easily configurable

Keep default payment-methods as mc,visa,amex
Merge pull request #3 from ICANS/make-paymentmethods-configurable

Make paymentMethods easily configurable