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

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
Merge pull request #83 from namin/init-dir

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