cmd/supercommand.go: log the version on startup

cmd/supercommand.go: log the version on startup

Often when going through debug logs, we end up trying to figure out what
version of what client on what architecture, etc. Often we have just enough
info to suss it out, but it is much nicer if it is just clearly written. This
adds it to supercommand so that if you are running in --debug or --show-log it
will be written for every command.

We already added this for jujud, I just missed adding it for juju itself, and I
ran into this today while trying to debug stuff for someone.

https://code.launchpad.net/~jameinel/juju-core/log-arch/+merge/211734

(do not edit description out of merge proposal)

[jameinel, 2014-03-19 14:11:15]

Please take a look.