Use optional env variable EMACS_LIVE_DIR to set emacs-user-directory.

Hi Sam,

My use case is that I'd like "emacs" to just be the vanilla emacs, but then, still be able to use emacs-live when I want to. 

I use this to have Emacs Live configured under a special user, say,
overtone. Then, from bash, I can run Emacs-Live as follows:
$ export EMACS_LIVE_DIR=~/../overtone/.emacs.d
$ emacs -u overtone

I am not sure whether this is useful for others too. What do you think?

Thanks. ~n