[Async] Enable async/await and update UIExplorer and tests

Summary:
- Enables async/await in .babelrc and transformer.js
- Adds regenerator to package.json. Users still need to explicitly require the regenerator runtime -- this is so that you only pay for what you use.
- Update AsyncStorage examples in UIExplorer to use async/await
- Update promise tests in UIExplorer to use async/await in addition to the promise API

Closes https://github.com/facebook/react-native/pull/1765
Github Author: James Ide <ide@jameside.com>

Libraries/JavaScriptAppEngine/Initialization/InitializeJavaScriptAppEngine.js

@@ -26,6 +26,7 @@
 require('RCTDebugComponentOwnership');
 require('RCTDeviceEventEmitter');
 require('PerformanceLogger');
+require('regenerator/runtime');
 
 if (typeof GLOBAL === 'undefined') {
   GLOBAL = this;