simplify messages: nick not needlessly resent.

previously added caching of data about others in order to colorize
feedback for other commands, well, that works for the nick as well.
add /clear command. make session per-tab instead of per-browser.

there's a bug in there someplace, having to do with the data cached
in the javascript side. occasional throws hinting that a doodles is
being destroyed twice. not that motivated right now to hunt it down.
i want to rewrite the javascript delta because it is getting too
convoluted, and maybe that will fix this bug.
added join/left messages. cleaned up redundant nicks. fixed user id bug.

more simplification of delta javascript. trimmed down html and css a bit.