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.
Not sure I get this.

Scenario: I'm issuing a PATCH request to update the title of the article and to create a new taxonomy term.

With the current code I would be:

  1. Limited to update operations (PATCH) for the taxonomy term.
  1. Updating the taxonomy term with the ID that corresponds to the article that we want to update. Therefore overwriting an unrelated taxonomy term with the data that we wanted for the new term.

I think that if we go down this path (and we should), we need to have a request that looks like this:

```php`
$request = array(
  'label' => 'parent',
  'body' => 'Gizra1',
  'er' => array(
    'endpoint' => 'api/v1/tag', // This would need to match the resource in public fields or fail.
    'method' => 'POST',
    'headers' => array(
      'X-Restful-Minor-Version' => 2,
    ),
    'request' => array(
      array(
        'label' => 'child',
        'body' => 'Gizra2',
      ),
      array(
        'label' => 'child',
        'body' => 'Gizra3',
      ),
    ),
  ),
);
$result = $handler->post('', $request);
```

Then we could be able to combine several requests as one.