Fixing some comments in the serializer tests to be accurate. Also tweaking the syntax of the 'dump' methods in the serializers to clarify that the serializers can acceptobjects other than just Hashes (even though PaperTrail will only pass attribute hashes to the serializers).
Making the built-in serializers declare 'extend self' so that their :load and :dump methods are available as module methods and instance methods.  This enables classes/modules that mix in one of these serializers to invoke or override the methods. Close #196