Emit terminal and iterm control paths to stderr.

Emit terminal and iterm control paths to stderr, rather than stdout, in order
to resolve issue with virtualenvwrapper's 'workon' command. Also change to
precmd hook, rather than chpwd hook. See reference:

http://stackoverflow.com/questions/18350280/python-virtualenvwrappers-workon-and-zsh/20050440#20050440?newreg=9f556e59a9004c32bab8c73e12030fde
https://github.com/wjv/oh-my-zsh/commit/d12113a9e89fed866db625f7e110fb62333b2746