Fixed Safari view not being presentable from a modal

[Amwam]

This is a fix for #26
The SafariView cannot be presented when a Modal is shown, as we try to present the SafariViewController on top of a view controller that is not visible.

This change cycles through any presented view controllers, and attempts to present on top of the foremost controller

Here is a minimal react-native application that presents this problem without the change, and is subsequently fixed with the change.
I have also tested the situation without the modal (simply by commenting out the <Modal> lines in this example)

import React, { Component } from 'react';
import {
  AppRegistry,
  View,
  Modal, 
  Button,
} from 'react-native';
import SafariView from 'react-native-safari-view'

export default class safaritest extends Component {
  render() {
    return (
        <View>
            <Modal>
                <Button title="Press me" onPress={()=> {
                    SafariView.isAvailable().then(()=> {
                        SafariView.show({url: 'http://github.com'})
                    })
                }} />
            </Modal>
        </View>
       );
  }
}


AppRegistry.registerComponent('safaritest', () => safaritest);

[Amwam]

@naoufal Any chance we can get some progress on this, or one of the other modal related PRs? (they all seem to have a similar solution)