[Image] Add scale support for base64-encoded image.

Summary:
Fix issue #1136
Closes https://github.com/facebook/react-native/pull/1721
Github Author: =?UTF-8?q?=E9=9A=90=E9=A3=8E?= <yinfeng.fcx@alibaba-inc.com>

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

React/Views/RCTTabBarItem.h

@@ -11,7 +11,7 @@
 
 @interface RCTTabBarItem : UIView
 
-@property (nonatomic, copy) NSString *icon;
+@property (nonatomic, copy) id icon;
 @property (nonatomic, assign, getter=isSelected) BOOL selected;
 @property (nonatomic, readonly) UITabBarItem *barItem;
 

React/Views/RCTTabBarItem.m

@@ -25,7 +25,7 @@
   return _barItem;
 }
 
-- (void)setIcon:(NSString *)icon
+- (void)setIcon:(id)icon
 {
   static NSDictionary *systemIcons;
   static dispatch_once_t onceToken;
@@ -54,7 +54,7 @@
   UIImage *image = [RCTConvert UIImage:_icon];
   UITabBarItem *oldItem = _barItem;
   if (image) {
-
+    
     // Recreate barItem if previous item was a system icon
     if (wasSystemIcon) {
       _barItem = nil;

React/Views/RCTTabBarItemManager.m

@@ -22,7 +22,7 @@ RCT_EXPORT_MODULE()
 }
 
 RCT_EXPORT_VIEW_PROPERTY(selected, BOOL);
-RCT_EXPORT_VIEW_PROPERTY(icon, NSString);
+RCT_EXPORT_VIEW_PROPERTY(icon, id);
 RCT_REMAP_VIEW_PROPERTY(selectedIcon, barItem.selectedImage, UIImage);
 RCT_REMAP_VIEW_PROPERTY(badge, barItem.badgeValue, NSString);
 RCT_CUSTOM_VIEW_PROPERTY(title, NSString, RCTTabBarItem)