Add Homebrew Cask as a package provider

Add Homebrew Cask[1] as a package provider. Allows for the installation
of DMG-based installers from the commandline, and by extension from
Boxen as a package provider.

[1]: http://caskroom.io/
[mattbostock] Add my Cask packages
Merge pull request #103 from alphagov/add-homebrew-cask-provider

Add Homebrew Cask provider