add support for disabled buttons and backButtonHidden on push

example/src/screens/SecondTabScreen.js

       buttons = [
         {
           title: 'Edit',
-          id: 'edit'
+          id: 'edit',
+          disabled: true
         },
         {
           icon: require('../../img/navicon_add.png'),

src/platformSpecific.ios.js

     passProps: passProps,
     style: navigatorStyle,
     backButtonTitle: params.backButtonTitle,
+    backButtonHidden: params.backButtonHidden,
     leftButtons: navigatorButtons.leftButtons,
     rightButtons: navigatorButtons.rightButtons
   });