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.
What about `get`? I think it would be nice to cover the following scenario:

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*

That's why I was suggesting the use of pseudo-API calls within the sub-resources. That way I think the code is way more maintainable since we will just have a single pattern to think of. We can have wicked functionality for free that is not supported anywhere else like authenticating different users in different sub-resources if necessary, having unlimited levels of sub-resources (create an article that references another article which has this image that in turn is tagged with a term…).

If we do custom logic for sub-resources at level-1 we will probably need to maintain those separately. I guess that the major concern is about DX in the client side, but maybe we can address that somehow later.

Other than that, this looks good.