One-to-one writable, nested serializer support

I recreated the one-to-one writable patch and applied it to the basic-nested-serialization branch.  Highlights:

* Notice the fix made to the NestedValidationError class for single nested objects.
* `.field_from_native` is very similar to the existing regular serializer method, but it includes the delete/update and save functionality.  It sticks to the pattern I've used of tracking the nested serializer objects and delete state.
* I haven't modified the tests any from how I originally wrote them.  They could be modified to mirror your style more if you want.