Add bundler

When I first cloned down your project I wasn't able to run the app because I didn't have the colorize gem installed. You can use [bundler](http://bundler.io/) to specify all of the dependencies for your project.

I recommend checking out the website for a more detailed description but basically you create a `Gemfile` in the root of your project, tell it to use https://rubygems.org as the source for installing the dependencies, then you list each of the gems that your project depends on. (as shown in the commits below).

It's reasonable to expect that most people will have the bundler gem already installed.

This way, people can pull down your project and run `bundle install` to install all of the gems that your project depends on.

You can merge this pull request into your project or set it up yourself for more practice.