message: Improve header buttons for Android
diff: import React from 'react';
  -import { Button } from 'react-native';
  +import { Button, Platform } from 'react-native';
  import { StackNavigator } from 'react-navigation';
  import LoginView from './views/login';
  import NewServerView from './views/serverNew';
  @@ -8,11 +8,13 @@ import RoomsListView from './views/roomsList';
  import RoomView from './views/room';
  +const position = Platform.OS === 'ios' ? 'headerLeft' : 'headerRight';
  +
  const MainCardNavigator = StackNavigator({
  Rooms: {
  screen: RoomsListView,
  navigationOptions: ({ navigation }) => ({
  - headerLeft: <Button title='Servers' onPress={() => navigation.navigate('ListServerModal')} />
  + [position]: <Button title='Servers' onPress={() => navigation.navigate('ListServerModal')} />
  })
  },
  Room: {
  @@ -38,19 +40,19 @@ export default new StackNavigator({
  Login: {
  screen: LoginView,
  navigationOptions: ({ navigation }) => ({
  - headerLeft: <Button title='Cancel' onPress={() => navigation.dispatch({ type: 'Navigation/BACK' })} />
  + headerLeft: Platform.OS === 'ios' && (<Button title='Cancel' onPress={() => navigation.dispatch({ type: 'Navigation/BACK' })} />)
  })
  },
  ListServerModal: {
  screen: ListServerView,
  navigationOptions: ({ navigation }) => ({
  - headerLeft: <Button title='Close' onPress={() => navigation.dispatch({ type: 'Navigation/BACK' })} />
  + headerLeft: Platform.OS === 'ios' && (<Button title='Close' onPress={() => navigation.dispatch({ type: 'Navigation/BACK' })} />)
  })
  },
  NewServerModal: {
  screen: NewServerView,
  navigationOptions: ({ navigation }) => ({
  - headerLeft: <Button title='Close' onPress={() => navigation.dispatch({ type: 'Navigation/BACK' })} />
  + headerLeft: Platform.OS === 'ios' && (<Button title='Close' onPress={() => navigation.dispatch({ type: 'Navigation/BACK' })} />)
  })
  }
  }, {
  