Testing is awesome

HI Colin!

I scanned your 1st little CLI project. Great job! I didn't run the project (mainly because I can't), but I scanned the code.  I added a Gemfile, and added something called Rspec (http://rubydoc.info/gems/rspec-core/frames). Ask one of your facilitators about it. It kicks ass.  

I added a few basic tests in Rspec that exposed a small bug in songs.rb. Bugs (or missing implementations) cause tests to fail.  I fixed the bug, and now the tests should pass. I did this as 3 commits so you can easily jump between them and see the tests pass/fail.

In practice, these 3 changes would have been squashed into one commit.