Need feedback: Add tests and organize

Hey - tried to add some tests and organize code here, and in the meantime, somebody coded up a Game of Life algo with a spec! So I figured I should get this PR up before going any further.

I like the repo, but I think you should have tests - I love working with tests when messing around with algorithms. 

I used Minitest for a few reasons:

* I'm not sure, but I think this repo could cater to beginners in Ruby. Minitest keeps things as simple as possible and doesn't require you to learn a new DSL to use.
* Minitest comes with performance testing out of the box (quite relevant for algorithms)
* Adds less weight to the gemfile (since Test::Unit is already in stdlib). 

So what do you think?