transform_attributes allows for arbitrary transforms on groups of attributes

Not sure, 
I'm good with `transform` but I think name is better, it matches with the `attribute` method (and avoids possible conflicts with that method) plus it's not necessarily the attribute of the model, take the example of
```Ruby
 transform_attributes [:date_iso8601], lambda { |name, model| model.send(name.to_s.split('_').first).iso8601 }
```
maybe `attribute_name` or `transformed_attribute_name`
wyt?