(maint) Add rubocop for testing

This commit adds the rubocop gem, a rubocop rake task and an initial rubocop
configuration file in prepartion for using rubocop in CI.

This commit also adds a rubocop todo file that exempts existing files from
violations.

Gemfile

@@ -22,6 +22,9 @@ group :test do
   gem 'rspec', '>= 3.2'
   gem 'simplecov', '>= 0.11.2'
   gem 'yarjuf', '>= 2.0'
+  # Rubocop would be ok jruby but for now we only use it on
+  # MRI or Windows platforms
+  gem "rubocop", :platforms => [:ruby, :x64_mingw]
 end
 
 # Evaluate Gemfile.local if it exists