didCreateRecord stores the new record in identity map

This allows for newly created records without an ID to be stored in the identity map, based on the ID that the server returns.

Before this PR

```javascript
user = App.User.create({ name: "john" });
user.save(); // returns { id: 1, name: "john" }

App.User.find(1) // GET /users/1.json
App.User.find(1) // loaded from the identity map
```

with this patch the record is already inserted in the identity map when it is created.