Add support for a parent field in bulk_index

* Adds a field on document objects that is removed before indexing that is used in the action as "_parent"
Prefer _parent to _parent_id

* Fixed docstring ,
* parent ID not value documented
* Feedback from @erikrose
Tolerate IDs and parent IDs that are 0 in bulk index.
Update README and documentation for bulk_index parent_field param.
Support _parent fields in bulk_index(). Fix #97.