Added internal HashMap serializer and support for capturing DataSerializ...

...able subclasses while serializing using standard Java Serialization, also added information on this to the documentation.

The later has speed implications if you use DataSerializable classes inside of classes using Java Serialization but actually it isn't possible anymore since DataSerializable does not extend Serializable since 3.2, so you get an  unexpected (probably non helpful exception NotSerializableException). Documentation includes the speed note and if somebody has the feeling that he wants to use it anyways he should have the option to do so :D