refactor($ionicScrollDelegate): make it a factory from current scope

BREAKING CHANGE: $ionicScrollDelegate no longer works globally; you must
create a new instance of each time you use it.  The actual methods on
each instance of $ionicScrollDelegate are the same, however.

Change your code from this:

```js
function MyController($scope, $ionicScrollDelegate) {
  $scope.scrollTop = function() {
    $ionicScrollDelegate.scrollTop();
  };
}
```

To this:

```js
function MyController($scope, $ionicScrollDelegate) {
  var delegate = $ionicScrollDelegate($scope);
  $scope.scrollTop = function() {
    delegate.scrollTop();
  };
}
```

js/ext/angular/src/directive/ionicContent.js

@@ -72,7 +72,7 @@ function($parse, $timeout, $ionicScrollDelegate, $controller, $ionicBind) {
     require: '^?ionNavView',
     scope: true,
     template:
-    '<div class="scroll-content">' +
+    '<div class="scroll-content" ng-class="$$contentState.getClassName()">' +
       '<div class="scroll"></div>' +
     '</div>',
     compile: function(element, attr, transclude) {