Handle international customer party id

In KSeF depending on customer location you need to fill in different
fields. For domestic customers we need to fill in the NIP number.
For customers inside the EU we need to fill in the UECode and UEVatNumber.
For non EU customers we need to fill in CountryCode and IDNumber and in
cases when there is no tax id number present we need to set the NoID flag.

parties.go

@@ -30,11 +30,11 @@ type ContactDetails struct {
 type Buyer struct {
 	NIP string `xml:"DaneIdentyfikacyjne>NIP,omitempty"`
 	// or
-	UECode      string `xml:"DaneIdentyfikacyjne>KodUE,omitempty"` //TODO
+	UECode      string `xml:"DaneIdentyfikacyjne>KodUE,omitempty"`
 	UEVatNumber string `xml:"DaneIdentyfikacyjne>NrVatUE,omitempty"`
 	// or
 	CountryCode string `xml:"DaneIdentyfikacyjne>KodKraju,omitempty"`
-	IDNumber    string `xml:"DaneIdentyfikacyjne>NrId,omitempty"`
+	IDNumber    string `xml:"DaneIdentyfikacyjne>NrID,omitempty"`
 	// or
 	NoID int `xml:"DaneIdentyfikacyjne>BrakID,omitempty"`
 
@@ -94,20 +94,19 @@ func NewBuyer(customer *org.Party) *Buyer {
 
 	buyer := &Buyer{
 		Name: customer.Name,
-		NIP:  string(customer.TaxID.Code),
 	}
 
-	if customer.TaxID.Country == l10n.PL {
+	if len(customer.TaxID.Code) == 0 {
+		buyer.NoID = 1
+	} else if customer.TaxID.Country == l10n.PL {
 		buyer.NIP = string(customer.TaxID.Code)
+	} else if isEUCountry(customer.TaxID.Country) {
+		buyer.UEVatNumber = string(customer.TaxID.Code)
+		buyer.UECode = string(customer.TaxID.Country)
 	} else {
-		if len(customer.TaxID.Code) > 0 {
-			buyer.IDNumber = string(customer.TaxID.Code)
-			buyer.CountryCode = string(customer.TaxID.Country)
-		} else {
-			buyer.NoID = 1
-		}
+		buyer.IDNumber = string(customer.TaxID.Code)
+		buyer.CountryCode = string(customer.TaxID.Country)
 	}
-	// TODO NrVatUE and UECode if applicable
 
 	if len(customer.Addresses) > 0 {
 		buyer.Address = newAddress(customer.Addresses[0])
@@ -128,6 +127,50 @@ func NewBuyer(customer *org.Party) *Buyer {
 	return buyer
 }
 
+func EUCountries() []l10n.CountryCode {
+	return []l10n.CountryCode{
+		l10n.AT, // Austria,
+		l10n.BE, // Belgium,
+		l10n.BG, // Bulgaria,
+		l10n.HR, // Croatia,
+		l10n.CY, // Republic of Cyprus,
+		l10n.CZ, // Czech Republic,
+		l10n.DK, // Denmark,
+		l10n.EE, // Estonia,
+		l10n.FI, // Finland,
+		l10n.FR, // France,
+		l10n.DE, // Germany,
+		l10n.GR, // Greece,
+		l10n.HU, // Hungary,
+		l10n.IE, // Ireland,
+		l10n.IT, // Italy,
+		l10n.LV, // Latvia,
+		l10n.LT, // Lithuania,
+		l10n.LU, // Luxembourg,
+		l10n.MT, // Malta,
+		l10n.NL, // Netherlands,
+		l10n.PT, // Portugal,
+		l10n.RO, // Romania,
+		l10n.SK, // Slovakia,
+		l10n.SL, // Slovenia,
+		l10n.ES, // Spain,
+		l10n.SE, // Sweden
+	}
+}
+
+func isEUCountry(country l10n.CountryCode) bool {
+	return contains(EUCountries(), country)
+}
+
+func contains(slice []l10n.CountryCode, item l10n.CountryCode) bool {
+	for _, value := range slice {
+		if value == item {
+			return true
+		}
+	}
+	return false
+}
+
 func addressLine1(address *org.Address) string {
 	if address.PostOfficeBox != "" {
 		return address.PostOfficeBox