Added support for bundler (Gemfile)

Makes it easier for others to contribute without having to hunt for dependencies to get the tests to pass. Ideally, this Gemfile would be just a `gemspec` declaration, but that would require `add_development_dependency` declarations in the gemspec. Unfortunately, the gemspec generation is coupled to the Rakefile and the library itself, which makes the addition of development dependencies worthy of a separate pull-request in itself.