Warn and exit when running inside tmux on OS X

tmux loses access to its original namespace on OS X which causes Chrome to
crash. See
https://github.com/ChrisJohnsen/tmux-MacOSX-pasteboard#interaction-with-tmux
and http://savanne.be/804-running-karma-and-chrome-under-tmux-on-osx
Merge pull request #31 from martinp/tmux-warning

Warn and exit when running inside tmux on OS X