run reattach-to-user-session in tmux on osx

In order to get pbcopy and pbpaste working under Mac OSX
and tmux, we need to run the reattach-to-user-session command

This commit automatically runs a custom script, reattach.sh,
on starting a tmux session, which tests for the exsistence
of the reattach-to-user-session command and runs it if available.

On systems where it is not available (e.g. Linux systems),
the script exists cleanly.