Dynamic Text Sizes for Text component

Summary:
Dynamic Text Sizes for Text component.
Text gains new prop - allowFontScaling (false by default).
There is also AccessibilityManager module that allows you to tune multipliers per each content size category.

React/Modules/RCTAccessibilityManager.h

@@ -0,0 +1,27 @@
+/**
+ * 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 <Foundation/Foundation.h>
+
+#import "RCTBridgeModule.h"
+#import "RCTBridge.h"
+
+extern NSString *const RCTAccessibilityManagerDidUpdateMultiplierNotification; // posted when multiplier is changed
+
+@interface RCTAccessibilityManager : NSObject <RCTBridgeModule>
+
+@property (nonatomic, readonly) CGFloat multiplier;
+
+@end
+
+@interface RCTBridge (RCTAccessibilityManager)
+
+@property (nonatomic, readonly) RCTAccessibilityManager *accessibilityManager;
+
+@end