Added pbcopy and pbpaste Mac OS X support.

I did this so that I could have pbcopy and pbpaste work in any new default
command based tmux windows I create. I did this after following the directions
outlined at the following,
http://robots.thoughtbot.com/post/19398560514/how-to-copy-and-paste-with-tmux-on-mac-os-x

There is also a similar explanation in the Prag Prog Tmux book.
Added session_scripts to customize my tmux sess.

I did this so that my tmux sessions for capture_api and codebreakdown would
both setup a tmux window with vim running in it.