add toggle tab bar navigation visibility

README.md

 });
 ```
 
+* **toggleNavBar(params = {})**
+
+Toggle whether the navigation bar is displayed or not.
+
+```js
+this.props.navigator.toggleNavBar({
+  to: 'hidden', // required, 'hidden' = hide navigation bar, 'shown' = show navigation bar
+  animated: true // does the toggle have transition animation or does it happen immediately (optional). By default animated: true
+});
+```
+
 ## Styling the navigator
 
 You can style the navigator appearance and behavior by passing a `navigatorStyle` object. This object can be passed when the screen is originally created; can be defined per-screen by setting `static navigatorStyle = {};` on the screen component; and can be overridden when a screen is pushed.

example/src/screens/ThirdTabScreen.js

   };
   constructor(props) {
     super(props);
+    this.state = {
+      navBarVisability: 'shown'
+    }
   }
   render() {
     return (
           <Text style={styles.button}>Show Modal Screen</Text>
         </TouchableOpacity>
 
+        <TouchableOpacity onPress={ this.onToggleNavBarPressed.bind(this) }>
+          <Text style={styles.button}>Toggle Navigation Bar</Text>
+        </TouchableOpacity>
+
       </View>
     );
   }
       screen: "example.ModalScreen"
     });
   }
+
+  onToggleNavBarPressed() {
+    this.state.navBarVisability = (this.state.navBarVisability === 'shown') ? 'hidden' : 'shown';
+    this.props.navigator.toggleNavBar({
+      to: this.state.navBarVisability,
+      animated: true  // true is default
+    });
+  }
+
+  componentDidUpdate() {
+    console.error('this is an error: ' + Math.random());
+    this.state.navBarState = 'shown';
+  }
+
 }
 
 const styles = StyleSheet.create({

src/Screen.js

     return platformSpecific.navigatorToggleTabs(this, params);
   }
 
+  toggleNavBar(params = {}) {
+    return platformSpecific.navigatorToggleNavBar(this, params);
+  }
+
   setTabBadge(params = {}) {
     return platformSpecific.navigatorSetTabBadge(this, params);
   }

src/platformSpecific.ios.js

   });
 }
 
-function navigatorSetTitleImage
-(navigator, params) {
+function navigatorSetTitleImage(navigator, params) {
   Controllers.NavigationControllerIOS(navigator.navigatorID).setTitleImage({
     titleImage: params.titleImage
   });
 }
 
+function navigatorToggleNavBar(navigator, params) {
+  Controllers.NavigationControllerIOS(navigator.navigatorID).setHidden({
+    hidden: ((params.to === 'hidden') ? true : false),
+    animated: params.animated
+  });
+}
+
 function navigatorToggleDrawer(navigator, params) {
   const controllerID = navigator.navigatorID.split('_')[0];
   if (params.to == 'open') {
   navigatorToggleDrawer,
   navigatorToggleTabs,
   navigatorSetTabBadge,
-  navigatorSwitchToTab
+  navigatorSwitchToTab,
+  navigatorToggleNavBar
 }