Enable iTerm2 tmux integration

Signed-off-by: Sorin Ionescu <sorin.ionescu@gmail.com>
Remove extra git-dir call in git-info and use git_dir variable instead
Create a new tmux session if none exists
Fix typo

Signed-off-by: Sorin Ionescu <sorin.ionescu@gmail.com>
[Fix #724] Unquote variable

Signed-off-by: Sorin Ionescu <sorin.ionescu@gmail.com>
Merge branch 'master' of git://github.com/sorin-ionescu/prezto