TabBarIOS tintColor

Summary:
[Origin Pull request](https://github.com/facebook/react-native/pull/961) from [cmcewen](https://github.com/cmcewen)

All the work have been done by @cmcewen, I just rebased his work with the master.
Closes https://github.com/facebook/react-native/pull/1337
Github Author: Stan Chollet <stanislas.chollet@gmail.com>

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

React/Views/RCTTabBar.m

@@ -114,6 +114,16 @@
   }];
 }
 
+- (void)setBarTintColor:(UIColor *)barTintColor
+{
+  _tabController.tabBar.barTintColor = barTintColor;
+}
+
+- (void)setTintColor:(UIColor *)tintColor
+{
+  _tabController.tabBar.tintColor = tintColor;
+}
+
 #pragma mark - UITabBarControllerDelegate
 
 - (BOOL)tabBarController:(UITabBarController *)tabBarController shouldSelectViewController:(UIViewController *)viewController