Migrate irb config files

Squashed commit of the following:

commit 1da79d0d845c21a7da8de3e53f569c28ce95d6d1
Author: T.S. Foster <hi@tsf.io>
Date:   Wed Sep 24 11:23:25 2014 +0100

    Add irbrc files
Add `.hushlogin` to home directory
Clarify `README.md`
Add tmux requirement
Rename `setup_dotfile_links` to `install_dotfiles`

Shorter and more descriptive