Make headers conform to package.el requirements, add autoloads

I'm adding a [Melpa](http://melpa.milkbox.net/) package recipe for nimrod-mode.el; this commit fixes the header/footer formatting so that `package-buffer-info` can process it. It also adds `autoload` cookies to the minor mode and `auto-mode-alist` entry, so that no user configuration is required when installing the file from a package repository.