Update for Django 1.3 Caching

Hi, 

I found your module and thought "wow, this is a really flexible implementation of simonw's ratelimit idea!"

However, when I tried using it with Django 1.3, I got strange results, the cache object would write the keys, but never read them again. This has to do with the way the cache object calls a default prefix function now, which includes your SITE_ID. 

In any case, these changes take that change into consideration. Let me know if there's anything I missed or, if there's a simpler solution to this issue.

Thanks, 
-Gavin