Replaced THEN-only `if' with `when'.

Replaced `if' forms with no ELSE forms with `when'.
Rewrote `elwiki/render-page'.

Rewrote the wrapper around `creole-wiki', using `creole-html'
directly.  `creole-wiki' is a wrapper around the core functions in
creole.el, and was too restrictive; using `creole-html' allows for
more flexibility.
with-stdout-to-elnode => elnode-send-html

Replaced all (with-stdout-to-elnode httpcon (princ ...)) forms
with (elnode-send-html httpcon ...) (not this *exact* replacement).
Fixed typo from previous commit

Added an unquote missing from commit 9da656b.
Added global stylesheet

Added a site-wide stylesheet, using the same styles from the output of
`creole-wiki'.  (This file was left out of commit 9da656b.)