Fix to add user as sender to invite_by_email method.

The OrganizationUserAddForm does not pass in the `request.user` as the
sender to the `invite_by_email` method. Therefore, the sender argument
resolves to `None`. An empty sender, in turn, creates an incomplete
invitation email body and defaults the reply_to to the
DEFAULT_FROM_EMAIL setting.
Updates authors [ci skip]
Update trove classifiers

This should help reflect in, e.g., djangopackages.com that django-organizations has PY3K support.
Add Python 3.4 to list