Stop extending if `String` flag is `false`

Allow `Ember.EXTEND_PROTOTYPES.String` is `false`.
Merge pull request #1319 from tricknotes/option-for-extending-string

Stop extending if `String` flag is `false`
Fix async belongsTo to return the record when data it's already loaded
Merge pull request #1320 from andrei1089/master

Fix async belongsTo to return the record when data it's already loaded
Polymorphic belongsTo attributes are camelized by the REST adapter.

This model:

```js
App.Comment = DS.Model.extend({
  blogMessage: DS.belongsTo('message' {
    polymorphic: true
  })
});
```

generates a JSON object like:

```js
{
  "blogMessage": 12,
  "blogMessageType": "post"
}
```
Add pushPayload method to store
fix typo
`didLoad` should fire even if the record isEmpty

Fixes #1317
Merge pull request #1309 from alexspeller/add-pushRaw-method

Add pushPayload method to store
Add test for many array state of new records
Update TRANSITION.md

Added documentation how to save relationships independently of it the model it belongs to (which could be achieved using transactions before).
Merge pull request #1325 from Cyril-sf/fix_rest_polymorphic_belongs_to

Polymorphic belongsTo attributes are camelized by the REST adapter.
Merge pull request #1329 from Globegitter/patch-2

Update TRANSITION.md
Merge pull request #957 from teddyzeenny/has-many-created

Set isLoaded flag for has many array of created records
Added transition docs for serialising polymorphic type
Extract polymorphic type now expects camelized "Type" key
to match serialisation
Merge pull request #1334 from bradleypriest/polymorphic-extract

Polymorphic extracting
Support embedded objects for DS.ActiveModelSerializer.serialize

This is a compatible way for [embedded
objects](http://emberjs.com/guides/models/defining-models/#toc_embedded-objects)
using `DS.ActiveModelSerializer`.

In 0.13:

``` javascript
DS.RESTAdapter.map('App.Post', {
  comments: { embedded: 'always' }
});
```

Using this patch:

``` javascript
App.PostSerializer = DS.ActiveModelSerializer.extend({
  attrs: {
    comments: { embedded: 'always' }
  }
});
```
Merge pull request #1336 from tricknotes/support-embedded-has-many

Support embedded objects for DS.ActiveModelSerializer.serialize
Defer ManyArray fetching

This avoids triggering a `fetchMany` if a later part of the payload
contains the necessary records. Fixes #1316.