[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>`. Only some headers are included based on what I've found to be useful.

Generated with:

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

React/React.h

@@ -0,0 +1,61 @@
+/**
+ * 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/React/Base/RCTAssert.h>
+#import <React/React/Base/RCTBridge.h>
+#import <React/React/Base/RCTBridgeModule.h>
+#import <React/React/Base/RCTCache.h>
+#import <React/React/Base/RCTConvert.h>
+#import <React/React/Base/RCTDevMenu.h>
+#import <React/React/Base/RCTEventDispatcher.h>
+#import <React/React/Base/RCTInvalidating.h>
+#import <React/React/Base/RCTJavaScriptExecutor.h>
+#import <React/React/Base/RCTJSMethodRegistrar.h>
+#import <React/React/Base/RCTKeyCommands.h>
+#import <React/React/Base/RCTLog.h>
+#import <React/React/Base/RCTRedBox.h>
+#import <React/React/Base/RCTRootView.h>
+#import <React/React/Base/RCTSparseArray.h>
+#import <React/React/Base/RCTTouchHandler.h>
+#import <React/React/Base/RCTUtils.h>
+#import <React/React/Executors/RCTContextExecutor.h>
+#import <React/React/Executors/RCTWebViewExecutor.h>
+#import <React/React/Views/RCTAnimationType.h>
+#import <React/React/Views/RCTAutoInsetsProtocol.h>
+#import <React/React/Views/RCTDatePickerManager.h>
+#import <React/React/Views/RCTMap.h>
+#import <React/React/Views/RCTMapManager.h>
+#import <React/React/Views/RCTNavigator.h>
+#import <React/React/Views/RCTNavigatorManager.h>
+#import <React/React/Views/RCTNavItem.h>
+#import <React/React/Views/RCTNavItemManager.h>
+#import <React/React/Views/RCTPicker.h>
+#import <React/React/Views/RCTPickerManager.h>
+#import <React/React/Views/RCTPointerEvents.h>
+#import <React/React/Views/RCTScrollableProtocol.h>
+#import <React/React/Views/RCTScrollView.h>
+#import <React/React/Views/RCTScrollViewManager.h>
+#import <React/React/Views/RCTShadowView.h>
+#import <React/React/Views/RCTSliderManager.h>
+#import <React/React/Views/RCTSwitch.h>
+#import <React/React/Views/RCTSwitchManager.h>
+#import <React/React/Views/RCTTabBar.h>
+#import <React/React/Views/RCTTabBarItem.h>
+#import <React/React/Views/RCTTabBarItemManager.h>
+#import <React/React/Views/RCTTabBarManager.h>
+#import <React/React/Views/RCTTextField.h>
+#import <React/React/Views/RCTTextFieldManager.h>
+#import <React/React/Views/RCTUIActivityIndicatorViewManager.h>
+#import <React/React/Views/RCTView.h>
+#import <React/React/Views/RCTViewControllerProtocol.h>
+#import <React/React/Views/RCTViewManager.h>
+#import <React/React/Views/RCTViewNodeProtocol.h>
+#import <React/React/Views/RCTWebView.h>
+#import <React/React/Views/RCTWebViewManager.h>
+#import <React/React/Views/RCTWrapperViewController.h>
+#import <React/React/Views/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 */,