Set auto-mode-alist at top-level and Add autoload cookie

This commit ensures that users who have installed `nginx-mode` from [MELPA](http://melpa.milkbox.net/) will be able to automatically turn on `nginx-mode` when visiting a `ngix.conf` or `/etc/nginx/.*` file without explicitly requiring the library first. For information about this fix, See [Packaging-Basics](http://www.gnu.org/software/emacs/manual/html_node/elisp/Packaging-Basics.html).