Incuna-videos

* Drop explicit support for django 1.5.
* Update requirements to latest versions.

* Allow VideoContent to be associated with multiple models without having reverse relation clashes. Consequently, the reverse relation to VideoContent is no longer accessible on Video.

*Apologies for the version inflation. I'm trying to be strict about SemVer.*

* Fix regression in extension of admin classes.

* **Rename the `subtitle` extension to `sub_heading`** to better reflect its purpose. The field added to `Video` keeps the name `sub_title` for the moment.
* **Introduce the `captions` extension.** This adds a `captions` FileField to Video that should reference a subtitles file. Captions are added to the `<video>` tag as a `<track>` tag where `src` points to the file.

* Remove requirement of legacy closed source `django-incuna` library.
* Preliminary python 3 support
* Tests!
* Remove `video` and `video_hours_count` tempate tags.
* Remove flowplayer and all javascript.
* OPEN SOURCE! WOO! (BSD 2-Clause License)
* Replace `sorl-thumbnail` with `django-imagekit`.
* Make "Chapter.seconds" return an int
* Add `VideoContent`
* Set `related_name` on `Source.video` and `Chapter.video`

**Known issues:**
* `Video.length_display` Is totally disabled -- this will get fixed in a later version
(See https://github.com/incuna/incuna-videos/issues/8)

**Backwards incompatible: may break project extensions.**

* Use feincms.extensions.ExtensionsMixin rather than incuna.utils.extensions.ExtensionsMixin
From FeinCMS v1.7 extensions sub class feincms.extensions.ExtensionsMixin.
Support for register(cls, admin_cls)-style functions is removed in FeinCMS v1.9.
* Convert all extension to sub class feincms.extensions.ExtensionsMixin.