Setup script

Adds a shell script called `script/setup`. The goal of this script is to help new developers check their system for compatibility, and walk them through the install process.

It only depends on `/bin/sh`, and can be safely run multiple times without creating adverse effects. It currently covers the first half of the install process (mainly the dependencies).

The motivation comes from this [maxim from Thoughtbot](http://wynnnetherland.com/linked/2013012801/bootstrapping-consistency):

> a developer should be able to clone the project and run a single, consistent, reliable command to start contributing

(This PR includes #3 and #4.)