Add package headers

Especially `Package-Requires` is necessary when slime is installed with package.el.

See Also
- http://www.gnu.org/software/emacs/manual/html_node/elisp/Simple-Packages.html
- http://www.gnu.org/software/emacs/manual/html_node/elisp/Library-Headers.html

Please add `Version` header if slime has version.