Support dynamic (lambda) namespace.

I know this is basically the same as https://github.com/redis-store/redis-store/pull/163 but it turns out I implemented this also. The extra value here are the more robust test and the caching of regex's in a hash instead of compiling them on each call.

In relation to @jodosha's comment about this needing to go into ActiveSupport and not into redis-store, my response is that this is already supported by ActiveSupport but breaks down in redis-store because lack of lambda support in redis-store. I think the patch is fairly clean and will enable lambda namespaces in redis-store in itself and will cause the lambda namespace in ActiveSupport to work correctly.

Specifically a call to a cache clear (flushdb) from ActiveSupport totally breaks down because this isn't supported in redis-store.