add a classHelper make component support for IE9+

Author: Saviio

src/js/angular-datepicker.js

@@ -305,23 +305,44 @@
           }
         };
 
+        $scope.classHelper={
+            add:function(element,klass){
+                if(element.className.indexOf(klass)>-1)
+                    return;
+                var classes=element.className.split(' ');
+                classes.push(klass);
+                element.className=classes.join(' ');
+            },
+            remove:function(element,klass){
+                if(element.className.indexOf(klass)==-1);
+                    return;
+                var classes=element.className.split(' ');
+                for(var i=0;i<classes.length;i++) if(classes[i]===klass){
+                        classes=classes.slice(0,i).concat(classes.slice(i+1));
+                        break;
+                }
+                element.className=classes.join(' ');
+            }
+        }
+
         $scope.showCalendar = function manageShowCalendar() {
           //lets hide all the latest instances of datepicker
           pageDatepickers = $window.document.getElementsByClassName('_720kb-datepicker-calendar');
 
           angular.forEach(pageDatepickers, function forEachDatepickerPages(value, key) {
-
-            pageDatepickers[key].classList.remove('_720kb-datepicker-open');
+            
+            pageDatepickers[key].classList ? pageDatepickers[key].classList.remove('_720kb-datepicker-open') : $scope.classHelper.remove(pageDatepickers[key],'_720kb-datepicker-open');
           });
 
-          theCalendar.classList.add('_720kb-datepicker-open');
+          theCalendar.classList ? theCalendar.classList.add('_720kb-datepicker-open') : $scope.classHelper.add(theCalendar,'_720kb-datepicker-open');
         };
 
         $scope.hideCalendar = function manageHideCalendar() {
-
-          theCalendar.classList.remove('_720kb-datepicker-open');
+          
+          theCalendar.classList ? theCalendar.classList.remove('_720kb-datepicker-open') : $scope.classHelper.remove(theCalendar,'_720kb-datepicker-open');
         };
 
+
         $scope.setDaysInMonth = function setDaysInMonth(month, year) {
 
           var i