[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>

react-packager/.babelrc

@@ -15,10 +15,12 @@
     "es6.properties.shorthand",
     "es6.spread",
     "es6.templateLiterals",
+    "es7.asyncFunctions",
     "es7.trailingFunctionCommas",
     "es7.objectRestSpread",
     "flow",
-    "react"
+    "react",
+    "regenerator"
   ],
   "sourceMaps": false
 }

transformer.js

@@ -28,10 +28,12 @@ function transform(srcTxt, filename, options) {
       'es6.properties.shorthand',
       'es6.spread',
       'es6.templateLiterals',
+      'es7.asyncFunctions',
       'es7.trailingFunctionCommas',
       'es7.objectRestSpread',
       'flow',
       'react',
+      'regenerator',
     ],
     sourceFileName: filename,
     sourceMaps: false,