Re-add Ubuntu support, maintained by the community

In #13 you mentioned that you removed Ubuntu support because you'd like to be able to do more OS X-specific stuff. However, these many months later everything you're doing is pretty cross-platform.

Another reason you mentioned dropping Ubuntu was because nobody at Thoughtbot uses Ubuntu. I'm not sure if this has changed, but what _has_ changed is how much the open source community loves and uses Laptop for their own computers.

Adding Ubuntu support back in, even with a clarification that it is community maintained and not officially supported or used by Thoughtbot, would be a wonderful thing. As an "olive branch" of peace between the two Unix operating systems, I've forked the repository, added an Ubuntu script (and an easy `zsh` installer), and updated the README. Would you please consider pulling these changes into your repo?