Support Alternative Django-DownloadView Views

[ryneeverett]

Part of my motivation for choosing django-downloadview over django-sendfile was the multiple options for file storage. (I now believe django-downloadview is better documented and maintained.)

However, by the time I finished the initial alpha release, I realized I'd completely coupled everything to the Download model's file field, so it wont be as simple as swapping out the view class. However, I think it can still be done by making Download an abstract base class, making a subclass for each downloadview view, and choosing them via a config option.