angular.module('auth')
    .factory('ValidationService', ['$http', function($http){
        var vs = {};
        var baseUrl = '/api/validate/';
        vs.validate = function(options) {
            var param = options.param,
                value = options.value,
                url   = options.url || baseUrl;
            if (param && value) {
                return $http.get(url, {params: {p: param, v: value}}).then(function(response) {
                    return response.data.isValid;
                });
            }
        };
	return vs;
    }]);

angular.module('auth')
    .directive('unique', function(ValidationService) {
        return {
            require: 'ngModel',
            scope: {
                param: '@unique'
            },
            link: function(scope, elm, attrs, ctrl) {
                ctrl.$asyncValidators.unique = function(modelValue, viewValue) {
                    var val = modelValue || viewValue;
                    return ValidationService.validate({param: scope.param, value: val});
                };
            }
        }
    });

<form novalidate class="css-form" name="usrform">
    Email: <input type="email" ng-model="user.email" name="email" unique="email" required/><br/>
</form>
<pre>email = "{{usrform.email.$error}}"</pre>