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.
> I want to create an article with title Title body Lorem ipsum, tagged by alpha, beta, gamma and associated to the entity with label Series: A

Not clear about what you say -- can you show how your ``$request`` would look like?

> anywhere else like authenticating different users in different sub-resources if necessary

IMO, Sounds like something that has no real use case ;)

> If we do custom logic for sub-resources at level-1 we will probably need to maintain those separately. 

Since this is a recursive function, you could nest those references. Maybe worth adding a test to assert it.