[StatusBar] Add ability to toggle the network activity indicator in the status bar + example in UIExplorer

Summary:
Added the ability to turn on and off the network activity indicator using:

```
StatusBarIOS.setNetworkActivityIndicatorVisible(true)
```
and
```
StatusBarIOS.setNetworkActivityIndicatorVisible(false)
```

Also added an example to the UIExplorer example app.

Fix #986
Closes https://github.com/facebook/react-native/pull/2079
Github Author: Mark Miyashita <negativetwelve@gmail.com>

Libraries/Components/StatusBar/StatusBarIOS.ios.js

@@ -35,6 +35,10 @@ var StatusBarIOS = {
     animation = animation || 'none';
     RCTStatusBarManager.setHidden(hidden, animation);
   },
+
+  setNetworkActivityIndicatorVisible(visible: boolean) {
+    RCTStatusBarManager.setNetworkActivityIndicatorVisible(visible);
+  },
 };
 
 module.exports = StatusBarIOS;