added accessor for serializer / deserializer / redis_factory

* added accessor for serializer / deserializer / redis_factory and its test
* 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.

As you have mentioned, working with Plack::Session::Store:CHI might be another way to achive this.
But it'll be re-implementation of your work on top of it because my only aim is
* to replace default serializer / deserializer
* to add flexibility to replace redis backends.