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/test/controller/ionicScrollController.unit.js

@@ -22,6 +22,17 @@ describe('$ionicScroll Controller', function() {
     });
   }
 
+  it('should set this.$scope', function() {
+    setup();
+    //Just an arbitrary way of checking that it is indeed a scope
+    expect(typeof ctrl.$scope.$id).toBe('string');
+  });
+
+  it('should set $scope.$$ionicScrollController', function() {
+    setup();
+    expect(ctrl.$scope.$$ionicScrollController).toBe(ctrl);
+  });
+
   it('should set this.element and this.$element', function() {
     setup();
     expect(ctrl.element.tagName).toMatch(/div/i);