Allow creating referenced sub-resources in a single request

#109 

```php
  $request = array(
    'label' => 'parent',
    'body' => 'Gizra1',
    'er' => array(
      'label' => 'child',
      'body' => 'Gizra2',
    ),
  );
  $result = $handler->post('', $request);
```

![site-install](https://cloud.githubusercontent.com/assets/125707/3604454/8c8aba5c-0d20-11e4-91af-63316fc0b6dd.jpg)

Validation is also working, since we are recursing into the referenced sub-resources, so nothing is saved if one of entities is invalid!

Note that the PR also moved request, account, path and method to properties of the handler, to stop passing so many arguments around from function to function.
Article:
  * title: _Title_
  * body: _Lorem ispum_
  * tags: _alpha, beta, gamma_
  * series_reference:
    - label: _Reference A_ (find it or create it)

> IMO, Sounds like something that has no real use case ;)

True, that's why I added the second example. This was just to point out that we will be creating a flexibility that we haven't even thought yet (think about contrib adding plugins and getting the ability to make them different for every subresource).

The following one sounds very realistic:

Article:
  * title: _Title_
  * body: _Lorem ispum_
  * related: (Article: you may want create, update, search, update or create, search or create)
    - title: _Related title_
    - image: (File entity: you may want create, update, search, update or create, search or create)
      - label: _Filename_
      - tags: (term: you may want create, update, search, update or create, search or create)
        - name: 'term1'
        - description: 'Lorem ipsum'
        - …

You can do this if you're able to treat each sub-resource as an original resource. Error handling becomes really simple, you just have to bubble up the error from the sub-resources.

Besides, we know that if you want to add an entity reference the user can do it with no problem with 2 requests:
   - 1st: Create related node doing: POST, `api/v2/related`, with headers `X-Restful-Minor-Version: 3`, …
  - 2nd: Create the article doing: PATCH, `api/v1/articles`, with headers `X-Restful-Minor-Version: 1`, …, with the node id from the step above.

My opinion is that if we have to create sub-resource operations with have to do it with the more intuitive way for the user: let them think that is just like doing multiple API calls, but save them the HTTP round trip by making them from within the backend server.