Uniqueness validator

- Supports :scope and :case_sensitive options
- Included en.yml locale file for testing, since the generated error messages
  are expected to be defined in a locale file (when passing in a symbol as the
  second parameter to errors#add)
  - See https://github.com/rails/rails/blob/v4.1.4/activemodel/lib/active_model/errors.rb#L381-L404
e.g. these tests still pass: https://github.com/brynary/minidoc/blob/master/test/validations_test.rb