Formulated acceptance spec for `embeds`
Allow to define models to embed on Collection

* Implemented DocumentModelMapper#embeds
* Implemented DocumentModelMapper#models_to_embed
* Implemented Collection.map
DocumentModelMapper#model_to_document supports embedded models
Explanation for document_to_model's handling of embedded models
Reek: Adjust nested block count to two
Nested Attributes: Feature done.
Merge pull request #14 from triAGENS/nested-attributes

Add nested attributes to models