This includes updates to work with Django 1.10, and bring the custom field code
more in line with a modern setup. This has the side effect of working filtering
by python date or datetime.
I've done a major version bump as the ApproximateDateField instance will now
return "" rather than None for an empty date, which is much more preferable
because you always had to query empty dates using "", not None. Unlikely to
break anything, given this behaviour was basically buggy, but seemed safest.
I've also added better testing via tox, and fixed any flake8 issues.
Thanks to John Lehmann and Aamir Adnan for reporting/tests.