Updated to use Ruby 2.0 in development and added Guard and Pry to development

Moved development dependencies out of the gemspec and into the Gemfile
#respond_to? calls are irrelevant. Just fail loudly if the message recipient doesn't support the expected method.  Besides, #respond_to? behaves differently WRT to protected in Ruby 2.0 http://tenderlovemaking.com/2012/09/07/protected-methods-and-ruby-2-0.html