emacs: ELPA compatibility

This commit makes `ninja-mode.el` installable with `package.el` which  is a package manager included in Emacs since version 24. 

- Add header line
- Add footer line
- Add Commentary section
- Add Code section

For information about this fix, See following GNU Emacs Manual:

- [Library-Headers](http://www.gnu.org/software/emacs/manual/html_node/elisp/Library-Headers.html)