Bring in pry and a couple of helpful plugins.

I really miss having some kind of debugger, and this is a good one. I didn't want to jump in and add my pet gems into the project, but it looks like Alex, Laura, Mike, and myself all use it already (they used pry-nav instead of pry-byebug, but pry-nav's documentation points you to byebug now).

These have helped me immensely with ActiveResource issues, and I'd like to get it into the official Gemfile instead of having to add it and then remember not to commit it every time.