Add serialization support (while still keeping backwards compatibility)

[aasaru]

I propose adding serialization support to PDU-s. I have tested it and it worked fine.

User can still break serialization by adding non-serializable object to a PDU using setReferenceObject(). It would be possible to change the method signature for this method so it would take only a serializable parameter but this would break backwards compatibility so I just added a comment.

The other way would be to deprecate current method and add new method, something like this:
/**
* @deprecated Replaced by {@link #setReference(Object)}
*/
@deprecated
public void setReferenceObject(Object value) {
this.referenceObject = value;
}

public <T extends Serializable> void setReference(T value) {
    this.referenceObject = value;
}