Installed debugger

Added and installed debugger gem.  See [Debugging with the debugger gem](http://guides.rubyonrails.org/debugging_rails_applications.html#debugging-with-the-debugger-gem).  Created :test and :development bundler groups and placed debugger and test-unit into those groups
But note @pellaea comment:
>I ... prefer minimizing the number of "moving parts" ... .  There's nothing I hate more when debugging something than to find that the reason I can't find the bug is because the code is behaving ever-so-subtly differently in the test environment! 

@pellaea suggests that if I want the debugger gem I should add it temporarily (via the gemfile and bundler update, then remove it).  I expect that I will be using that gem all the time, so I'd really like to have it there permanently.