Django-postgres-copy

* ``mapping`` argument to ``copy_from`` now optional with header crosswalk inferred (magnunleno)
* Added support for optional ``delimiter`` argument to ``copy_to`` (magnunleno)
* Added support for optional ``header`` argument to ``copy_to`` (ckirby)
* Added support for optional ``null`` string argument to ``copy_to`` (ckirby)
* Refactor field resolution to use Django's preferred ``model._meta.get_field`` method (ckirby)
* More consistent use and testing of ``FieldDoesNotExist`` error (palewire)

* Added ``delimiter`` option to ``copy_to``.

* Added multi-database support to ``copy_to``.

* Added ``copy_to`` for export support
* Added new manager for easier implementation
* Added ``copy_from`` to manage to connect existing import methods to the new idiom

* Field retrieval now looks for matches with field.name and field.attname

* Added optional hooks before and after copy and insert steps
* Removed vestigal ``copy_type`` attribute on transformations
* Python 3.6 testing
* Django 1.11 testing