Some nicer command-validation logic.

(This also gets rid of the "waiting" error, which is unneeded
because the commands that are unavailable while waiting are also
unavailable while offline for the same reason.)
Fix #40: Turn nick spaces into underlines.

xmpp doesn't like spaces in nicks. The current way cadence did this
was crap, because the url-encoded %20s were visible in external clients.
Instead, these are now turned into underscores which work everywhere.

(Also, added a check to /nick that requires a non-empty nickname.)
Add a /version command.

This command links to the current version on github
(provided it exists in the @cburschka/cadence repository).

Also, the Makefile now always regenerates .sed.script,
in order to make config.js contain the latest version string.