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>