Added a few wrapper scripts to work around OSX bullshit

Rather than add a bunch of platform-specific tweaks and changes to my
dotfiles I'm trying out using wrapper scripts instead. This OSX + tmux
clipboard fiasco is a giant PITA too. WTF, Apple?

Why oh why didn't I just request a Lenovo?
Changed browser to Chrome
Replaced custom tree alias with call to tree
Added shell functions to perform a countdown or a timer