Bump fetch and add exports/globals for Headers, Request and Response

Summary:
Now on fetch 0.8.1, the latest tagged release. Previous version used was 0.7.0. See #1162 cc @vjeux @jtremback
Closes https://github.com/facebook/react-native/pull/1192
Github Author: Brent Vatne <brent.vatne@madriska.com>

Test Plan: I arc patched and ran movies demo and storyline, they work fine

Libraries/JavaScriptAppEngine/Initialization/InitializeJavaScriptAppEngine.js

@@ -135,7 +135,12 @@ function setupXHR() {
   // The native XMLHttpRequest in Chrome dev tools is CORS aware and won't
   // let you fetch anything from the internet
   GLOBAL.XMLHttpRequest = require('XMLHttpRequest');
-  GLOBAL.fetch = require('fetch');
+
+  var fetchPolyfill = require('fetch');
+  GLOBAL.fetch = fetchPolyfill.fetch;
+  GLOBAL.Headers = fetchPolyfill.Headers;
+  GLOBAL.Request = fetchPolyfill.Request;
+  GLOBAL.Response = fetchPolyfill.Response;
 }
 
 function setupGeolocation() {

Libraries/JavaScriptAppEngine/Initialization/loadSourceMap.js

@@ -17,8 +17,6 @@ var RCTSourceCode = require('NativeModules').SourceCode;
 var SourceMapConsumer = require('SourceMap').SourceMapConsumer;
 var SourceMapURL = require('./source-map-url');
 
-var fetch = require('fetch');
-
 function loadSourceMap(): Promise {
   return fetchSourceMap()
     .then(map => new SourceMapConsumer(map));