Added Screen to module exports

src/index.js

@@ -1,7 +1,9 @@
 import Navigation from './Navigation';
+import Screen from './Screen';
 import {NavigationToolBarIOS} from 'react-native-controllers';
 
 module.exports = {
   Navigation,
-  NavigationToolBarIOS
+  NavigationToolBarIOS,
+  Screen
 };