Tmux iTerm2 integration

This adds functionality to enable iTerm2 tmux integration as documented [here](https://code.google.com/p/iterm2/wiki/TmuxIntegration).

Essentially, if ZSH_TMUX_ITERM2 is set to true, the tmux plugin adds the option `-CC` to `tmux` whenever it is invoked. Enabling this also disables the loading of any "$HOME/.tmux.conf" configuration file, as they are not yet supported.

This works with ZSH_TMUX_AUTOSTART; however, I have noticed one bug, likely upstream with iTerm2, where a quick close and re-open of an iTerm2 tmux session, where the message "A tmux protocol error occurred. Reason: %begin with empty command queue" occurs, but I may be wrong.