Implement draggable annotations on MapView. Closes #2512

Summary: Closes https://github.com/facebook/react-native/pull/4441

Reviewed By: svcscm

Differential Revision: D2707897

Pulled By: nicklockwood

fb-gh-sync-id: 6f67f711c1ec1f821d03b9b1ea5cc39859d28fd1

Examples/UIExplorer/ImageExample.js

@@ -442,7 +442,8 @@ exports.examples = [
     title: 'Image Size',
     render: function() {
       return <ImageSizeExample source={fullImage} />; 
-    }
+    },
+    platform: 'ios',
   },
 ];