* added accessor for serializer / deserializer / redis_factory and its tests
* t/02-storable.t tests with Storable serializer / deserializer if it's available.
* t/03-message-pack.t tests with Data::MessagePack serializer / deserializer if it's available.
* redis_factory will be useful when you want to replace Redis.pm with other modules which
  work with different redis backends like hiredis.