Added capability to handle multi-level nested embedded documents.
Given getItem()'s ability to recursively resolve a model:

* There's no need to loop in higher levels.
* Plain models are allowed where:
  * A simple string `selector` can be passed to `scrapy.scrape()`
  * and/or an object with the `selector` property to get an anonymous value.
Merge pull request #2 from eeshi/feature/multilevel

New: node-scrapy accepts plain strings and multi-level documents as model.