Refactor header, tintColor support, redesign iOS back button (#109)

examples/NavigationPlayground/js/SimpleStack.js

@@ -58,6 +58,7 @@ const MyProfileScreen = ({ navigation }) => (
 MyProfileScreen.navigationOptions = {
   header: ({ state, setParams }) => ({
     title: `${state.params.name}'s Profile!`,
+    tintColor: '#007AFF',
     // Render a button on the right side of the header.
     // When pressed switches the screen to edit mode.
     right: (