embedded table: use DOMAIN to look up the resource fields

This patch resolve two issues:

it's now possible to have a resource field that is a relation to another relation, even if the name doesn't match. For example, product_stable and product_devel are two one-to-one field with a relationship to product
when we associate a foreign resource, we let eve.utils.embedded_document() decide what field should be import in the embedded resource. Previously, all the fields of the embedded resource were imported, with the thread of potential endless loops. For example, a any-to-one relation called foo with an associate to bar that we pull with embedded. bar has got a foo_collection with a link to foo. Previously, the foo_collection filed was automatically added in the list of field of bar. This was enough to create a resource loop in lookup_foreign_resource.
eve.utils.embedded_document() was confused because it was try to resolve itself embedded resource that were already pulled by eve_sqlalchemy.utils.sqla_object_to_dict(). Because of that, all the embedd resources were prefixed with a ID_FIELD in the final JSON. We now only retrieve the ID_FIELD value, as expected by embedded_document()
the patch, with https://github.com/nicolaiarocci/eve/pull/656 will allow the use of sub-embedded document

pyeve / eve-sqlalchemy