Add advanced remote return value checking

I had an API that wasn't able to return a "true" or "false" string for the remote validator. So I added a method to allow more advanced checking of a remote's response. It would be used like this:

```javascript
    $('.registration-form').validate({
        rules: {
            user_email: {
                required: true,
                email: true,
                // Use a standard remote object to initialize the call
                remote: {
                    url: "/the/url",
                    beforeSend: function() {
                        // Show a spinner
                    },
                    complete: function() {
                        // Hide the spinner
                    }
                }
            }
        },
        messages: {
            // The default error message will be used during failure
            user_email: { remote: "This email account already exists!" }
        },
        /* New functionality: This code will be executed in the success
         * method of jQuery's ajax call. It should return true or false
         * based on whatever criteria you need
         */
        validators: {
            user_email: {
                remote: function(response) {
                        if (response.complex_value meets your criteria) {
                            return true;
                        } else {
                            return false;
                        }
                    }
                }
            },
        }
});```