Add a task for uploading nginx site config and a generator for configuring template before uploadinging

add a task ```cap ningx:config``` to upload nginx site config
add a generator ```rails g capistrano:nginx_puma:config``` for customizing template before uploading 
update readme
normally it should be /opt/nginx/sites-availables, but to covert all weird settings, we can fetch for :nginx_sites_available_path, wdyt ?