Add ability to validate survey question results on server via Ajax requests

[andrewtelnov]

Create an API that will allow easily to call server code to validate question results.

[andrewtelnov]

onServerValidateQuestions callback is added into survey class. Here is the example of using:

<script>
    var json = {
        questions: [{ type: "text", name: "country" }]
    };
    var survey = new Survey.Survey(json);
    //assign call to onServerValidateQuestions callback
    survey.onServerValidateQuestions = function (survey, options) {
        //options.data contains the data for the current page.
        var countryName = options.data["country"];
        //If the question is empty then do nothing
        if (!countryName) options.complete();
        //call the ajax method
        $.ajax({
            url: "http://services.groupkt.com/country/get/all"
        }).then(function (data) {
            var found = false;
            var countries = data.RestResponse.result;
            for (var i = 0; i < countries.length; i++) {
                if (countries[i].name == countryName) {
                    found = true; break;
                }
            }
            //if the country is unknown, add the error
            if (!found) options.errors["country"] = "The country name is unknown.";
            //tell survey that we are done with the server validation
            options.complete();
        });
    }
    survey.render("dxsurvey");
</script>