The fields are defined by a single object where each key represents the
name of one field or many fields separated per a comma

New argument in the constructor to pass a object with custom messages
for each field and their corresponding rules

The messages must to be defined in the "fields" argument

The setRules() method was updated with the new way of define fields

Was added two methods to handle the callbacks when the form is
submitted. The callback in the constructor is an "always" callback but
it is optional