Add a readOnly option to DS.attr

This adds a readOnly option to DS.attr which causes the attribute not to be serialized to json. 
I'm not sure if readOnly is the right name because it doesn't stop you changing the value in any way it just makes sure the changes aren't persisted.

My main use case is for created_at and updated_at fields. I'm currently filtering them out of the json in my Rails controllers at the server end. But it would be nice to not send them at all.