Embedding in subdocuments

Feature allowing to embed documents located in nested dicts and lists. It solves the problem described here: https://github.com/nicolaiarocci/eve/issues/222
Please review the code, let me know if I should amend something and sorry for my English in the docs.

It’s possible now to embed documents by references located in
subdocuments using dot notation.
For example, it’s possible to make the query:
?embedded={“user.friends”:1} for a document which have “user”
subdocument with list of references “friends”.