Set auto-mode-alist with autoload-cookie

This commit ensures that users who have installed `actionscript-mode` with `package.el` will be able to automatically turn on `actionscript-mode` when visiting a `as`  file without explicitly requiring the library first. For details, See [Packaging-Basics](http://www.gnu.org/software/emacs/manual/html_node/elisp/Packaging-Basics.html).