MailGun's Guardpost email validation directive for AngularJS - How to create Async directives with $validators in Angular >= v1.3.0-beta.16

lgGuardpost.js

'use strict';
/* jshint camelcase: false */
angular.module('lgValidation')
  // Uses the Guardpost API from MailGun to validation the
  // email addresses and provide suggestions
  .directive('lgGuardpost', function($http, $document,
        MAILGUN_BASE_URL, MAILGUN_API_KEY) {
    return {
      require: 'ngModel',
      link: function(scope, element, attrs, ctrl) {
        var isValid = false; // invalid value (e.g. incorrect email)
        var validatedValue; // the value that has been validated

        ctrl.$validators.guardpostCheck = function(value) {
          // Is valid if the current value has been validated
          return value === validatedValue;
        };

        ctrl.$validators.guardpost = function(value) {
          // Prevent the validator from running twice
          if (value !== validatedValue) {
            validatedValue = value;
            // Only run this validator if the email syntax is valid
            if (!ctrl.$error.required && !ctrl.$error.email) {
              // Guardpost doc:
              // http://documentation.mailgun.com/api-email-validation.html
              $http.get(MAILGUN_BASE_URL + '/v2/address/validate', {
                params: {
                  address: value,
                  api_key: MAILGUN_API_KEY
                }
              }).success(function(data, status, headers, config) {
                // Check is the view value hasn't changed. If it is the case
                // an other validation is already in progress and this one
                // doesn't matter anymore.
                if (ctrl.$viewValue === config.params.address) {
                  if (status === 200) {
                    if (data.is_valid === true) {
                      // Email is valid
                      isValid = true;
                    } else {
                      // Email is not valid
                      isValid = false;
                      // If a suggestion is present as part of the response,
                      // add it to the model to be displayed
                      ctrl.suggestion = data.did_you_mean ?
                      data.did_you_mean : null;
                    }

                  } else {
                    // Fail safe, consider email valid on failure.
                    isValid = true;
                  }
                  ctrl.$validate();
                }
              }).error(function(data, status, headers, config) {
                // Fail safe, consider email valid on failure
                if (ctrl.$viewValue === config.params.address) {
                  isValid = true;
                  ctrl.$validate();
                }
              });
            }
          }
          // Is valid if the current value has been validated && is valid
          return (value === validatedValue) && isValid;
        };

      }
    };
  });

how do i use this?