Add the Homebrew Cask library
Add the function ‘brew_cask_install_or_update_binary’ to install or update binaries with Homebrew Cask
Add binaries to provision script for Homebrew Cask:

- Install GitX-dev
- Install VirtualBox
- Install Vagrant
- Install Spectacle
Update README:

- Split the provisioning section up in categories
- Add the essential Homebrew Cask to the README
- Add the binaries GitX-dev, VirtualBox, Vagrant and Spectacle to the README