Tell OS X Terminal about the current directory

So that, if the setting is enabled, new and restored tabs can open in
the same directory.

Fix found via thanpolas/dotfiles-1@ee74a53c843, and adapted for ZSH
compat by @evansolomon.
Merge pull request #55 from markjaquith/issue/persist-pwd-in-osx-terminal

Tell OS X Terminal about the current directory