Add support for delete animation in LayoutAnimation on iOS

Summary:This adds support for delete view animations in LayoutAnimation for iOS. It supports the same properties as the create animation (alpha, scale).

This allows making simple animations when removing a view which is normally hard to do in React since we need to not remove the view node immediately.

**Test plan**
Tested add/removing views in the UIExample explorer with and without setting a LayoutAnimation. Also tested that the completion callback still works properly. Tested that user interation during the animation is properly disabled.

![layout-anim2](https://cloud.githubusercontent.com/assets/2677334/14595471/86fb1654-050d-11e6-8b38-fe45cc2dcd71.gif)

I also plan to work on improving the doc for LayoutAnimation as well as making this PR for android too.
Closes https://github.com/facebook/react-native/pull/6779

Differential Revision: D3215525

Pulled By: sahrens

fb-gh-sync-id: 526120acd371c8d1af433e8f199cfed336183775
fbshipit-source-id: 526120acd371c8d1af433e8f199cfed336183775

Examples/UIExplorer/UIExplorerList.android.js

@@ -138,6 +138,10 @@ const APIExamples = [
     key: 'LinkingExample',
     module: require('./LinkingExample'),
   },
+  {
+    key: 'LayoutAnimationExample',
+    module: require('./LayoutAnimationExample'),
+  },
   {
     key: 'LayoutExample',
     module: require('./LayoutExample'),