Django-postgres-copy

* Added ability to write to file-like objects

* Added `copy_to` options for `encoding`, `escape`, `quote` and `force_quote`.

* To realize speed gains, added new `ConstraintsManager` to drop all indexes and constraints on the database prior to loading.

* Added support for Django 2.0
* Dropped testing for Django versions prior to 1.11 and psycopg versions prior to 2.7.3, though they still likely work

* ``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``.