Add read-only option to Store.attr(), Store.hasMany() and Store.hasOne()

[haydn]

The needs to be support for read-only attributes:

var store = new Store();
store.define("products", {
  commentCount: Store.attr({ readOnly: true })
});

Attributes that are read only are fetch from the API, but can't be updated.

[haydn]

This should also be available for hasOne() and hasMany().