We can speed up things for the supported types by keeping the code in the
way it was.
We can also avoid to loop trough all serializers in the deserialization by
trying to access the class already in the Hash.
We could also speed up the custom serialization if we define the class
that is going to be serialized when registering the serializers, but
that will remove the possibility of defining a serialzer for a
superclass and have the subclass serialized using it.
Now custom serialziers can register itself in the serialized hash using
the "_aj_serialized" key that constains the serializer name.
This way we can avoid poluting the hash with many reserved keys.
