Some minor doc changes.

See: M-x checkdoc RET.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Add autoload cookie.

See (info "(elisp) Autoload").  This is needed to generate proper
autoloads (e.g., for el-get or elpa).

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Properly clean up when buffer is killed.

When the buffer is killed (e.g., by C-x k) the timers are not cleaned up
properly.  By adding a `kill-buffer-hook' this can be prevented.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Fail when svg is not supported

Without this check, the timer functions continue to
run (and throw errors) even though the `slime-volleyball`
function fails to set up and create the buffer.
Merge branch 'minorchanges' of git://github.com/ruediger/slime-volleyball
Merge branch 'patch-2' of git://github.com/purcell/slime-volleyball
Fix some formatting issues
README.md: Mention librsvg requirement

As suggested by Raimon Grau, https://github.com/kidd
Add (require 'cl) fallback for Emacs versions < 24.3
README.md: Add a screenshot

As suggested by Jory Graham, https://github.com/jory
Mention SVG in description