Fix file header for ELPA compatibility

Hi, I plan to add this to [MELPA](http://melpa.milkbox.net/). This commit makes `elm-mode` installable with `package.el`.

- Add package description (Feel free to change description if desired.)
- Add footer line
- Add Commentary section
- Add Code section
- Add licence

For information about this fix, See following GNU Emacs Manual:

- [Simple-Packages](http://www.gnu.org/software/emacs/manual/html_node/elisp/Simple-Packages.html)
- [Library-Headers](http://www.gnu.org/software/emacs/manual/html_node/elisp/Library-Headers.html)

(Tip: if you enable auto-insert-mode, Emacs will offer to insert the standard elisp boilerplate when you create a new
.el file.)

Cheers,

-Yasuyuki