Adding pry-rails to Gemfile for debugging

Pry is a powerful interactive console that makes it easy to browse source and debug. See: http://pryrepl.org/

@AudreyAltman and @markbreedlove should probably check this branch out and run ```rails console``` to make sure they don't hate pry before merging.  I use it constantly, but will defer if people don't want it popping open on them.