Add Style enforcer (via Rubocop)

It will fail the build, but as it is currently,
most of the cops are 'todos'. Great for new contributors.. :)

test/fixtures/poro.rb

@@ -83,7 +83,7 @@ Location = Class.new(Model)
 Place    = Class.new(Model)
 Tag      = Class.new(Model)
 VirtualValue = Class.new(Model)
-Comment  = Class.new(Model) do
+Comment = Class.new(Model) do
   # Uses a custom non-time-based cache key
   def cache_key
     "#{self.class.name.downcase}/#{self.id}"