Add recipe for slime-volleyball.

Truly impressive graphical game for Emacs.

* http://www.fitzsim.org/blog/?p=132
* https://github.com/fitzsim/slime-volleyball

Currently it requires manual autoloads until my PR #1 is accepted.  But
right now the repo can't be changed due to Lisp-In-Summer contest rules.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>