WIP: 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.
Exactly, that's when using the API via Drupal. When using it via CURL we'll have the request object in `php://input` so we can read it in the constructor if necessary.