Django-tables2

- Feature: Pinned rows ([411](https://github.com/bradleyayers/django-tables2/pull/411) by [djk2](https://github.com/djk2), fixes [#406](https://github.com/bradleyayers/django-tables2/issues/406))
- Fix an issue where `ValueError` was raised while using a view with a `get_queryset()` method defined. (fix with [423](https://github.com/bradleyayers/django-tables2/pull/423) by [desecho](https://github.com/desecho))

- Fix URLS to screenshots in on PyPi description (fixes [ 398](https://github.com/bradleyayers/django-tables2/issues/398))
- Prevent superfluous spaces when a callable `row_attrs['class']` returns an empty string ([417](https://github.com/bradleyayers/django-tables2/pull/417) by [Superman8218](https://github.com/Superman8218)), fixes [#416](https://github.com/bradleyayers/django-tables2/issues/416))

- Return `None` from `Table.as_values()` for missing values. [419](https://github.com/bradleyayers/django-tables2/pull/419)
- Fix ordering by custom fields, and refactor `TableData` [424](https://github.com/bradleyayers/django-tables2/pull/424), fixes [#413](https://github.com/bradleyayers/django-tables2/issues/413)
- Revert removing `TableData.__iter__()` (removed in [this commit](https://github.com/bradleyayers/django-tables2/commit/8fe9826429e6945a9258bc181fcbd711b282dba9)), fixes [#427](https://github.com/bradleyayers/django-tables2/issues/427), [#361](https://github.com/bradleyayers/django-tables2/issues/361) and [#421](https://github.com/bradleyayers/django-tables2/issues/421).

- Implement method `Table.as_values()` to get it's raw values. [394](https://github.com/bradleyayers/django-tables2/pull/394) by [intiocean](https://github.com/intiocean)
- Fix some compatibility issues with django 2.0 [408](https://github.com/bradleyayers/django-tables2/pull/409) by [djk2](https://github.com/djk2)

- Documentation for `None`-column attributes [401](https://github.com/bradleyayers/django-tables2/pull/401) by [dyve](https://github.com/dyve)

- `None`-column attributes on child class overwrite column attributes of parent class [400](https://github.com/bradleyayers/django-tables2/pull/400) by [dyve](https://github.com/dyve)