[Headers] Add an umbrella header for the library

The umbrella header allows projects that link React Native using CocoaPods to import it in standard fashion: `#import <React/React.h>`. Currently all of the headers are exported so that external projects can experiment.

Generated with:

    find React -name "*.h" | awk -F'/' '{print "#import <React/"$NF">"}' > React/React.h

React/React.h

@@ -0,0 +1,72 @@
+/**
+ * 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 <React/RCTAssert.h>
+#import <React/RCTBridge.h>
+#import <React/RCTBridgeModule.h>
+#import <React/RCTCache.h>
+#import <React/RCTConvert.h>
+#import <React/RCTDevMenu.h>
+#import <React/RCTEventDispatcher.h>
+#import <React/RCTInvalidating.h>
+#import <React/RCTJavaScriptExecutor.h>
+#import <React/RCTJSMethodRegistrar.h>
+#import <React/RCTKeyCommands.h>
+#import <React/RCTLog.h>
+#import <React/RCTRedBox.h>
+#import <React/RCTRootView.h>
+#import <React/RCTSparseArray.h>
+#import <React/RCTTouchHandler.h>
+#import <React/RCTUtils.h>
+#import <React/RCTContextExecutor.h>
+#import <React/RCTWebViewExecutor.h>
+#import <React/Layout.h>
+#import <React/RCTAlertManager.h>
+#import <React/RCTAppState.h>
+#import <React/RCTAsyncLocalStorage.h>
+#import <React/RCTExceptionsManager.h>
+#import <React/RCTSourceCode.h>
+#import <React/RCTStatusBarManager.h>
+#import <React/RCTTiming.h>
+#import <React/RCTUIManager.h>
+#import <React/React.h>
+#import <React/RCTAnimationType.h>
+#import <React/RCTAutoInsetsProtocol.h>
+#import <React/RCTDatePickerManager.h>
+#import <React/RCTMap.h>
+#import <React/RCTMapManager.h>
+#import <React/RCTNavigator.h>
+#import <React/RCTNavigatorManager.h>
+#import <React/RCTNavItem.h>
+#import <React/RCTNavItemManager.h>
+#import <React/RCTPicker.h>
+#import <React/RCTPickerManager.h>
+#import <React/RCTPointerEvents.h>
+#import <React/RCTScrollableProtocol.h>
+#import <React/RCTScrollView.h>
+#import <React/RCTScrollViewManager.h>
+#import <React/RCTShadowView.h>
+#import <React/RCTSliderManager.h>
+#import <React/RCTSwitch.h>
+#import <React/RCTSwitchManager.h>
+#import <React/RCTTabBar.h>
+#import <React/RCTTabBarItem.h>
+#import <React/RCTTabBarItemManager.h>
+#import <React/RCTTabBarManager.h>
+#import <React/RCTTextField.h>
+#import <React/RCTTextFieldManager.h>
+#import <React/RCTUIActivityIndicatorViewManager.h>
+#import <React/RCTView.h>
+#import <React/RCTViewControllerProtocol.h>
+#import <React/RCTViewManager.h>
+#import <React/RCTViewNodeProtocol.h>
+#import <React/RCTWebView.h>
+#import <React/RCTWebViewManager.h>
+#import <React/RCTWrapperViewController.h>
+#import <React/UIView+React.h>

React/React.xcodeproj/project.pbxproj

@@ -164,6 +164,7 @@
 		58114A4F1AAE93D500E7D092 /* RCTAsyncLocalStorage.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = RCTAsyncLocalStorage.h; sourceTree = "<group>"; };
 		58C571BF1AA56C1900CDF9C8 /* RCTDatePickerManager.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = RCTDatePickerManager.m; sourceTree = "<group>"; };
 		58C571C01AA56C1900CDF9C8 /* RCTDatePickerManager.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = RCTDatePickerManager.h; sourceTree = "<group>"; };
+		78BEE2481ABABC2E00F44561 /* React.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = React.h; sourceTree = "<group>"; };
 		830213F31A654E0800B993E6 /* RCTBridgeModule.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = RCTBridgeModule.h; sourceTree = "<group>"; };
 		830A229C1A66C68A008503DA /* RCTRootView.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = RCTRootView.h; sourceTree = "<group>"; };
 		830A229D1A66C68A008503DA /* RCTRootView.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = RCTRootView.m; sourceTree = "<group>"; };
@@ -334,6 +335,7 @@
 		83CBBA2F1A601D0F00E9B192 /* React */ = {
 			isa = PBXGroup;
 			children = (
+				78BEE2481ABABC2E00F44561 /* React.h */,
 				83CBBA491A601E3B00E9B192 /* Base */,
 				134FCB381A6E7F0800051CC8 /* Executors */,
 				13B07FC41A68125100A75B9A /* Layout */,