Fix file headers and footer for ELPA compatibility, add autoloads

We've received a [request](milkypostman/melpa#873) to add pig-mode to [MELPA](http://melpa.milkbox.net/) -- this commit fixes the formatting of the file so that it conforms to the standard expected by `package.el`. It also adds autoload cookies for the major mode and `auto-mode-alist` entry.