[TabBarIOS] Add translucent property to TabBarIOS

Summary:
To be on par with NavigatorIOS, I added the translucent property to TabBarIOS.

Usage:
```
<TabBarIOS
 translucent={false} // default is true
/>
```
Closes https://github.com/facebook/react-native/pull/1937
Github Author: Jean Regisser <jean.regisser@gmail.com>

React/Views/RCTTabBar.h

@@ -15,6 +15,7 @@
 
 @property (nonatomic, strong) UIColor *tintColor;
 @property (nonatomic, strong) UIColor *barTintColor;
+@property (nonatomic, assign) BOOL translucent;
 
 - (instancetype)initWithEventDispatcher:(RCTEventDispatcher *)eventDispatcher NS_DESIGNATED_INITIALIZER;
 

React/Views/RCTTabBar.m

@@ -140,6 +140,14 @@ RCT_NOT_IMPLEMENTED(-initWithCoder:(NSCoder *)aDecoder)
   _tabController.tabBar.tintColor = tintColor;
 }
 
+- (BOOL)translucent {
+  return _tabController.tabBar.isTranslucent;
+}
+
+- (void)setTranslucent:(BOOL)translucent {
+  _tabController.tabBar.translucent = translucent;
+}
+
 #pragma mark - UITabBarControllerDelegate
 
 - (BOOL)tabBarController:(UITabBarController *)tabBarController shouldSelectViewController:(UIViewController *)viewController

React/Views/RCTTabBarManager.m

@@ -23,5 +23,6 @@ RCT_EXPORT_MODULE()
 
 RCT_EXPORT_VIEW_PROPERTY(tintColor, UIColor)
 RCT_EXPORT_VIEW_PROPERTY(barTintColor, UIColor)
+RCT_EXPORT_VIEW_PROPERTY(translucent, BOOL)
 
 @end