$("#register-form").validate({
  rules: {
    username: {
      required: true,
      minLength: 2,
      remote: "register.php"
    }
  },
  messages: {
    username: {
      required: "Enter your username",
      minLength: "At least 2 characters are necessary",
      remote: String.format("The name {0} is already in use")
    }
  }
});