Implement Blob support for XMLHttpRequest

Summary:
This PR is a followup to https://github.com/facebook/react-native/pull/11417 and should be merged after that one is merged.

  1. Add support for creating blobs from strings, not just other blobs
  1. Add the `File` constructor which is a superset of `Blob`
  1. Add the `FileReader` API which can be used to read blobs as strings or data url (base64)
  1. Add support for uploading and downloading blobs via `XMLHttpRequest` and `fetch`
  1. Add ability to download local files on Android so you can do `fetch(uri).then(res => res.blob())` to get a blob for a local file (iOS already supported this)

  1. Clone the repo https://github.com/expo/react-native-blob-test
  1. Change the `package.json` and update `react-native` dependency to point to this branch, then run `npm install`
  1. Run the `server.js` file with `node server.js`
  1. Open the `index.common.js` file and replace `localhost` with your computer's IP address
  1. Start the packager with `yarn start` and run the app on your device

If everything went well, all tests should pass, and you should see a screen like this:

![screen shot 2017-06-08 at 7 53 08 pm](https://user-images.githubusercontent.com/1174278/26936407-435bbce2-4c8c-11e7-9ae3-eb104e46961e.png)!

Pull to rerun all tests or tap on specific test to re-run it

  [GENERAL] [FEATURE] [Blob] - Implement blob support for XMLHttpRequest
Closes https://github.com/facebook/react-native/pull/11573

Reviewed By: shergin

Differential Revision: D6082054

Pulled By: hramos

fbshipit-source-id: cc9c174fdefdfaf6e5d9fd7b300120a01a50e8c1

Libraries/Blob/__mocks__/BlobModule.js

@@ -0,0 +1,17 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @flow
+ * @format
+ */
+const BlobModule = {
+  createFromParts() {},
+  release() {},
+};
+
+module.exports = BlobModule;

Libraries/Blob/__mocks__/FileReaderModule.js

@@ -0,0 +1,21 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @flow
+ * @format
+ */
+const FileReaderModule = {
+  async readAsText() {
+    return '';
+  },
+  async readAsDataURL() {
+    return 'data:text/plain;base64,NDI=';
+  },
+};
+
+module.exports = FileReaderModule;