Add enums of accepted values for fields like Country and Currency Code

[Samael]

while testing the API I've found that I cannot use "GBP" as a currency code and I cannot specify "United States" as a country (seems "US" is the expected value). The fields are both strings with no indication of how the format of the values should be. Perhaps it would be better to be able to use .billingAddressCountry(CountryCode.US) and .preferredCurrencyCode(CurrencyCode.USD).

[cb-yateshmathuria]

@SingingBush - Thank you for suggestion. You can always refer our API Docs for any clarification.

For billingAddressCountry please see this. Under billingAddress attributes you can find country and expected values
For preferredCurrencyCode please see this.