WebSocket polyfill

Summary:
- Added as a library in /Libraries/WebSocket
- Drag and drop to add to project (similar to adding Geolocation polyfill)
- Exposed as `window.WebSocket` which conforms with https://developer.mozilla.org/en-US/docs/Web/API/WebSocket specs
Closes https://github.com/facebook/react-native/pull/890
Github Author: Harrison Harnisch <hharnisc@gmail.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Libraries/WebSocket/RCTWebSocketExecutor.h

@@ -0,0 +1,22 @@
+/**
+ * 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.
+ */
+
+#import "RCTDefines.h"
+
+#if RCT_DEV // Debug executors are only supported in dev mode
+
+#import "RCTJavaScriptExecutor.h"
+
+@interface RCTWebSocketExecutor : NSObject <RCTJavaScriptExecutor>
+
+- (instancetype)initWithURL:(NSURL *)URL NS_DESIGNATED_INITIALIZER;
+
+@end
+
+#endif