adding functionality for adding an object to a prong.
implemented functionality for adding pair to a prong.  refactored unittests for the adding pair nodes.
implemented _get_effective_parent_type() and refactored code to use it.
implemented func to add array to prong
minor refactoring
Added JsonLiteralValueNode and partially implemented its add() method.
implemented JsonStringNode.add().  refactored PairNode.add() to use a JsonLiteralValue object as a key.