Implement a postMessage function and an onMessage event for webviews …

Summary:
JS API very similar to web workers and node's child process.

Work has been done by somebody else for the Android implementation over at #7020, so we'd need to have these in sync before anything gets merged.

I've made a prop `messagingEnabled` to be more explicit about creating globals—it might be sufficient to just check for an onMessage handler though.

![screen shot 2016-09-06 at 10 28 23](https://cloud.githubusercontent.com/assets/7275322/18268669/b1a12348-741c-11e6-91a1-ad39d5a8bc03.png)
Closes https://github.com/facebook/react-native/pull/9762

Differential Revision: D4008260

fbshipit-source-id: 84b1afafbc0ab1edc3dfbf1a8fb870218e171a4c

ReactAndroid/src/main/java/com/facebook/react/uimanager/UIManagerModuleConstants.java

@@ -77,6 +77,7 @@ import com.facebook.react.uimanager.events.TouchEventType;
         .put("topLoadingFinish", MapBuilder.of("registrationName", "onLoadingFinish"))
         .put("topLoadingStart", MapBuilder.of("registrationName", "onLoadingStart"))
         .put("topSelectionChange", MapBuilder.of("registrationName", "onSelectionChange"))
+        .put("topMessage", MapBuilder.of("registrationName", "onMessage"))
         .build();
   }