Add autoload cookie for ELPA installation

Hi, I plan to add this to [MELPA](http://melpa.milkbox.net/). This commit ensures that users who have installed `org-grep` from [MELPA](http://melpa.milkbox.net/) will be able to use the code 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).