Django-popup-view-field

* Support for Django 2.0 and 2.1

* Added PopupViewModelField that returns a model instance instead of text

* Added `attrs` attribute to PopupViewField which is passed to the Widget

* Added `callback_data` attribute to PopupViewField (read more in README)

* Support for Django 1.11

* Remove PopupViewField from django_popup_view_field.__init__,
now you must import PopupViewField from django_popup_view_field.fields

* Sorting imports using isort

* Checking the order of imports in the tox

* Checking support for django-bootstrap3 v8.1.0

* Added django-bootstrap3 v8.1.0 to tox

* Adding version in django_popup_view_field.__init__

* Initial version + tests + Travis CI