Adding Django Cache Option

This adds an option that lets you fall back to the django cache if you don't want to use redis directly. Doesn't require django, but provides the option.

I didn't spend a ton of time on tests or documentation since I'm not 100% sure how you'd like to go forward with either of these. It isn't an especially complex addition and there aren't tests for the redis implementation so I think we might be OK there, but documentation wise I think there might be room for more beyond the small bit I added and the information in the cache's docstring.

Thanks