Recursive transformations

This is my proposed solution for applying transformations on value (i.e returning `datetime` objects for `created_at` and `modified_at`). It should also scale to creating actual resource objects from attributes.

An example:

```python
[Product({
 'created_at': datetime.datetime(2014, 5, 10, 22, 43, 56),
 'currency': u'SEK',
 'description': u'This is a test product.',
 'id': u'9cVh',
 'images': [],
 'modified_at': datetime.datetime(2014, 5, 10, 23, 8, 17),
 'price': 0,
 'price_includes_tax': True,
 'quantity': 1,
 'slug': u'tictail-python-test-product',
 'status': u'published',
 'store_id': u'KGu',
 'title': u'Tictail Python Test Product',
 'unlimited': False,
 'variations': [{
  u'created_at': datetime.datetime(2014, 5, 10, 22, 43, 56),
  u'id': u'dCxc',
  u'modified_at': datetime.datetime(2014, 5, 10, 23, 8, 17),
  u'quantity': 1,
  u'title': None,
  u'unlimited': False
 }]
})]
```

@kalasjocke and @tiwilliam what do you think?