Tri-form

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* Added useful return value on `Form.apply(...)`

* Improve repr on `Field`

* Added `Form.editable` settings for setting all fields at once

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* Bugfix for endpoint dispatch: we must allow a result to be an empty list/dict

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* Added `Link` class, and redefined the submit button on the form in terms of a `Link`

* Fixed incorrect handling of template rendering and `RequestContext` between django before and after 1.10

* Dropped official support of django 1.7

* Added `attrs` member on `Form`, and added method `render_attrs` that you use to render the attributes into html

* Added six as a dependency

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* Fixed `render_attrs` to escape quotation marks

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* Fixed non-editable fields so that you can override the rendering

* Fixed bug where raw_data was sent to render_value when there was an error on the field

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* `Field` class now inherits from `object`, making the implementation more pythonic.
(Attributes still possible to override in constructor call, see `NamespaceAwareObject`)

* Added `Form.extra` namespace for custom data.