API Improvements
- Updated to [Django REST Framework](http://django-rest-framework.org) 2.X series
- Better modular directory structure ([rest](http://wq.io/docs/rest), [patterns](http://wq.io/docs/about-patterns), and [contrib.files](http://wq.io/docs/files))
- Full read-write nested serializer for `annotate.Annotation`
- Allow [annotate](http://wq.io/docs/annotate)'s `Annotation` to be swapped with custom models
- Various other improvements