Use cid until server returns "new" uniqe id for this model. 

It seems that if model does not have an id then `backbone.localStorage` adapter generate `new` `cid` for it. But when the server returns a new id it `takes over` `model.id` and then this makes the adapter save the model twice. Any ideas how to make it play nice when waiting for `id` from server but still allow offline support?
