[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>

Examples/UIExplorer/UIExplorerIntegrationTests/js/IntegrationTestsApp.js

@@ -10,6 +10,8 @@
  */
 'use strict';
 
+require('regenerator/runtime');
+
 var React = require('react-native');
 
 var {