Remove the now unused splash logo.

Also, allow passing extra arguments through to
configure in calref-install.sh: To wit, these are

--cdn-prefix <path>
--cdn-url <url>
--prefix <path>

(The last one isn't needed and defaults to CWD.)
Install in debug mode, preserving JS and CSS.

The extra bandwidth is minimal, but it allows much easier diagnosis
for Javascript problems that may occur.