NavigatorIOS custom nav bar colors

Summary:
NavigatorIOS with custom nav bar custom colors, working example included in
UIExplorer.

Based on pull request #318 to complete pending work based on comments.
Closes https://github.com/facebook/react-native/pull/695
Github Author: Eduardo <edo.lomeli@gmail.com>

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

Examples/UIExplorer/Navigator/NavigatorExample.js

@@ -175,4 +175,6 @@ var styles = StyleSheet.create({
   }
 });
 
+TabBarExample.external = true;
+
 module.exports = TabBarExample;