Update `Unmarshal` method for the message and the rest of the fields

[alovak]

It should be possible to use native types in data structures like this when we extract data from the message into the struct using Unmarshal:

// define a struct for the authorization request
type AuthorizationResponse struct {
    MTI                  string `index:"0"`
    STAN                 string `index:"11"`
    // ...
}

// usage
request := &AuthorizationResponse{}

message := iso8583.NewMessage(Spec)
err := message.Unpack(someBinaryData)
// handle error
err = message.Unmarshal(request)
// handle error

// request.MTI => "0110"
// request.STAN => "000123"

It should include the following:

• Message
• field.String -> supported target types string, int (+pointers)
• field.Numeric -> supported target types string, int (+pointers)
• field.Binary -> supported target types are []byte, string, *string. If target isstring, we should convert the value into hex
• field.Hex-> same as field.Binary

the reference implementation is here and it includes update of the message.Unmarshal (src) and field.String/Unmarshal (src):

[alovak]

closed by #291