Added about_list_values.rb for Ruby 2.0

Hi. Isaac (irosenb) and I are interns at the Flatiron School; Avi Flombaum (aviflombaum) has e-mailed the Koans maintainers regarding our desire to add to the Koans. In this branch, I've added about_list_values.rb, which demonstrates how #values_at can be used for Arrays and Hashes. I'll write more about this and we'll make pull requests for other branches as we isolate features (we accidentally added them to a single branch at first, but to make maintenance easier, we'll give each new file or feature its own branch).