Improve payment means with UNCL4461

[TSimkus]

Changes:

• Added full list of UNCL4461 (Payment Means) codes.
• Updated <cac:PaymentMeans> use UNCL4461 codes
• Updated ->setPaymentMeansCode() to take string instead of int
• Updated ->getPaymentMeansCode() to return string instead of int
• Updated unit test to use UNCL4461 codes

This is an extension for PR#54

[TecsiAron]

A lot more elegant than my hack, thanks!