add CachedConfig and CachedAllConfig

These new classes cache the config (key, value) pairs for CONSTANCE_CACHE_TIMEOUT seconds in
Python itself. This helps for situations where the constance config is called very often.

The default timeout is 1 second. This might seem very short but these classes where build
for high traffic sites with a lot of requests per second. However there is nothing stopping
you from using a larger timeout if that's more applicable.