ELPA compatibility

This commit makes `debug-print.el` installable with package.el.

- fix header line
- add URL header
- add Package-Requires header for using lexical-binding

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)