Add decorate method to ModelDecorator class

-----FAILING TEST-----

1) ModelDecorator#decorate adds a uuid to a model
   Failure/Error: uuid               = ModelDecorator.new.decorate(model_without_uuid)
   ArgumentError:
     wrong number of arguments (1 for 0)
Simplify test for ModelDecorator#response

-----FAILING TEST-----

1) ModelDecorator#response returns model with uuid
   Failure/Error: end
   NoMethodError:
     undefined method `match' for nil:NilClass
Green:  ModelDecorator#response returns model with uuid

- deleted the decorate method
- All ModelDecorator tests passing
Refactor ModelDecorator tests
Refactor ModelDecorator class
Set SignUpsController#index to use ModelDecorator on SignUp
Red:  ModelDecorator#response overrides models to_param method

-----FAILING TEST-----

1) ModelDecorator#response overrides models to_param method
   Failure/Error: model_decorator.override_to_param_method
   NoMethodError:
     undefined method `override_to_param_method' for #<ModelDecorator:0x007f987ef63330>
Green:  ModelDecorator#response overrides models to_param method