Add :mapper_class option
Rails 4.0.4, Ruby 2.0.0
Bumped RUby to 2.1.2 and Rails to 4.0.5
690

Make specs pass with Rspec 3
Do not raise if target is an empty association

Task 690
Merge remote-tracking branches 'origin/690_rspec3', 'origin/mapper_class_option' and 'origin/rails_4'