Sitemap generator feature

## Overview
There 2 approaches:

1. make a `sitemaps_controller` and generate the correspond xml format sitemap on the fly.
2. a scheduled task which generate the sitemap.xml on the `public` folder, if you're on Heroku which you don't have the permission for that, you can upload to some cloud services or any other server to host the file.

I prefer the 2nd one, don't like to pollute my controller with this "backend" task.

## Issues
Somehow my AWS account is not available for now so I have to look for alternatives, ended up with my VPS server.

I implemented a `SftpAdapter` for sitemap_generator to do this.