Add nightly option to download the develop branch from GitHub

As I've been playing around with Laravel 5, I've been wanting a quick way to install the new directory structure.  This pull request should handle that.

Also, this is my first pull request ever.  I got the idea after watching the [Laracast video](https://laracasts.com/series/how-to-build-command-line-apps-in-php/episodes/4) recreating this command line tool.  I hope it's useful.