Export `Navigator` class for public use (#1647)

* Make Navigator Public

* Make Screen.js export both Screen and Navigator

* Update Navigation.js to import Screen correctly

* Make Navigator public for Android too

* Forgot to export after importing

* ESLint fix

* ESLint fix

* ESLint Fix

* ESLint Fix

src/Navigation.js

 import React from 'react';
 import {AppRegistry} from 'react-native';
 import platformSpecific from './deprecated/platformSpecificDeprecated';
-import Screen from './Screen';
+import {Screen} from './Screen';
 
 import PropRegistry from './PropRegistry';
 

src/Screen.js

   }
 }
 
-export default class Screen extends Component {
+class Screen extends Component {
   static navigatorStyle = {};
   static navigatorButtons = {};
 
     }
   }
 }
+
+export {
+  Screen,
+  Navigator
+};

src/deprecated/indexDeprecated.android.js

 import SharedElementTransition from './../views/sharedElementTransition';
 import NativeEventsReceiver from './../NativeEventsReceiver';
 import ScreenVisibilityListener from './../ScreenVisibilityListener';
+import {Navigator} from './../Screen';
 
 module.exports = {
   Navigation,
   SharedElementTransition,
   NativeEventsReceiver,
-  ScreenVisibilityListener
+  ScreenVisibilityListener,
+  Navigator
 };

src/deprecated/indexDeprecated.ios.js

 import SharedElementTransition from '../views/sharedElementTransition';
 import NativeEventsReceiver from './../NativeEventsReceiver';
 import ScreenVisibilityListener from './../ScreenVisibilityListener';
+import {Navigator} from './../Screen';
 
 module.exports = {
   Navigation,
   NavigationToolBarIOS,
   SharedElementTransition,
   NativeEventsReceiver,
-  ScreenVisibilityListener
+  ScreenVisibilityListener,
+  Navigator
 };