Add nested create for 1to1 reverse relationships

This pull request supersedes #536. It aims to get basic functionality of nested create for 1to1 reverse relationships including validation support.  Obviously the logic will become more complicated when update/delete functionality is added.  Pending discussion and merging of this functionality, I believe we should then focus on:

* Add update/delete functionality for 1to1 reverse relationships
* Add support for one-to-many by adding list deserialization support