keys/iter_keys: support for REVERSE_KEY_FUNCTION setting

django provides a way to use a custom key function via KEY_FUNCTION
setting. To provide keys/iter_keys functionality properly redis_cache
now has a way to support custom revese_key function, that reverses
the key back.
REVERSE_KEY_FUNCTION: move parameter to CACHES settings

It seems more appropriate to have REVERSE_KEY_FUNCTION as an
optional parameter next to KEY_FUNCTION than as a top-level parameter
REVERSE_KEY_FUNCTION: tests

added a test to check if a custom key function actually works
also fixed behaviour for old(1.4.x) django
Merge pull request #81 from teferi/reverse_key_function

Add support for REVERSE_KEY_FUNCTION setting.