Update cache configuration

1. `CACHE_*` settings are deprecated in Django 1.3 and have been
removed in Django 1.5. The new way is the `CACHES` setting.
2. Adding the KEY_PREFIX parameter which can be overwritten by
local_settings.py so that each server using the same Django instance
can have a unique prefix for its caches
Adding the KEY_PREFIX parameter

Allowing each server to have a unique prefix for its caches