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.
If I understand correctly what you mean, it will change this:

```php
$handler = restful_get_restful_handler('test_articles', 1, 2);
$handler->post('', $request);
$handler->get($entity_id);
```

Into this:
```php
$handler = restful_get_restful_handler('test_articles', 1, 2, 'post', $request);
// POST
$result = $handler->post();

// GET, with the same handler.
$handler->setRequest(NULL);
$handler->setPath($entity_id);
$result = $handler->get();
```

The latter seems a bit less readable to me, but I'm open to discussion if you feel strongly about it.