adding MEMCACHE_KEY_PREFIX parameter

Adding the MEMCACHE_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
Merge pull request #714 from g76r/f_keyprefix

adding KEY_PREFIX parameter to set a memcached key prefix
Merge branch 'master' into log_all_exceptions