title: Problems with react-navigation exactly with TabNavigator and StackNavigator
body: <p>hi i have a problem with react-navigator exactly TabNavigator and StackNavigator .
  the problem is that, the StackNavigator does't work when i put it inside a TabNavigator </p>
  
  <p>I have this part in App.js, in this part I declared  my stackNavigator</p>
  
  <pre><code>import uno  from './android/Componentes/uno'
  import dos  from './android/Componentes/dos'
  import tres  from './android/Componentes/tres'
  import inde from './android/Componentes/inde'
  import Bonton from './android/Componentes/Bonton'
  class App extends Component{
   render(){
    return(
      &lt;inde /&gt;
  );
  }
  }
  const Root = StackNavigator ({
    inde:{screen:inde},
    uno:{screen: uno},
    dos:{screen: dos},
    tres:{screen: tres},
    Bonton:{screen:Bonton}
  });
  
  export default Root;
  </code></pre>
  
  <p>in this part (inde.js) i declare my TabNavigator </p>
  
  <pre><code> import uno  from './uno'
   import dos  from './dos'
   import tres  from './tres'
  
   export default class inde extends Component{
     render(){
       return(
         &lt;Root /&gt;
       );
     }
   }
   const Root = TabNavigator ({
      uno:{screen: uno},
      dos:{screen: dos},
      tres:{screen: tres}
    });
  </code></pre>
  
  <p>and this in uno.js </p>
  
  <pre><code>export default class uno extends Component{
  render(){
      return(
          &lt;Button
      onPress={() =&gt; this.props.navigation.navigate('Bonton')}
      title="Go to Lucy's profile"
    /&gt;
      );
  }
  }
  </code></pre>
  
  <p>am i doing anything wrong?</p>