Better support for read-only attributes sent back by the server

When a record is fetched from the server, any attributes the server
returns that are not declared on the Spine model are preserved and
accessible on the client side. Examples might include Rails' :created_at
and :updated_at fields.

However, if a record is created client-side and saved to the server,
Spine does not allow those additional undeclared attributes to get set
on the record.

While I haven't yet come up with a solution to the problem, I wanted to open a dialog to discuss potential options. One suggestion is to have the Ajax responder through the incoming data over to the model's @refresh method, rather than trying to call @updateAttributes on the instance. Thoughts?