Initial attempt at a juju command-line tool
Features worthy of comment:
* Partly inspired by $GOROOT/src/cmd/go, but eschews IMO-nasty globals in
favour of testability.
* Uses rog's gnuflag package because it's convenient and correct and no
harder than using the builtin flag package.
* I don't like the way we have 2 code paths that handle errors and usage-
printing, but since flag insists on using stderr (and gnuflag rightly
preserves this behaviour) I think it's necessary.
* `go build -o juju` inside cmd/juju should produce a binary you can play
around with; this is probably the best way to actually explore how it
works and whether it's functionally acceptable (as opposed to
stylistically, technically, etc).
* Includes a not-really-functional implementation of juju.Conn, which I
can't actually use (or test effectively) until we have environ types
(including "dummy") registered globally; but alows me to demonstrate
how I vaguely intend a Command should work.
R=rog, niemeyer
CC=
https://codereview.appspot.com/5554075