Bootstrap project

*I found this project on 24 Pull Requests, so I thought the additions in this PR will help future contributors get started with the project quickly.* See [Bootstrapping consistency](http://wynnnetherland.com/linked/2013012801/bootstrapping-consistency) for more information about project bootstrapping.

First, I added `cmd: 'bundle exec rspec'` to `Guardfile` so that on file change, the specs are run. In this file, I also removed unnecessary comments and spaces.

I created a `/script` directory with four Bash executables:
 - `script/bootstrap` — bootstraps the project so that it's ready for development (in this case, it just quietly runs `bundle install`)
 - `script/console` — bootstraps the project, then runs `bundle console`
 - `script/guard` — bootstraps the project, then runs Guard
 - `script/test` — bootstraps the project, then runs the specs (via `bundle exec rspec`, like in Guard)

Finally, I added a "Development" section at the bottom of `README.md` swiftly talking about each of the scripts and how/why they benefits the user.

*Note that I didn't remove the existing spec-running Rake task (that's up to you).*