Check if the current_auth has the `:read_posts` scope, then use `Post.first(:id => params.id)

(See [tentd/api/posts.rb#L29-L54](https://github.com/jonasschneider/tentd/blob/7f00b324dd2bb94173bf0bb6f830de1ea7703794/lib/tentd/api/posts.rb#L29-L54))