Encapsulate persisted logic into a Model method

Right now Backbone.dualStorage infers a model is persisted if it's id
length is not 36 characters.

With this change applications can decide how to compute if a model is
persisted or not (checking for a `created_at` attribute for example),
allowing the server to use UUIDs as well.
Merge pull request #83 from tute/master

Encapsulate persisted logic into a Model method