ENH: Adding functions for the node referencing mechanism. See http://viewvc.na-mic.org/viewvc.cgi/Slicer4/trunk/Libs/MRML/Core/vtkMRMLNode.h?view=log&pathrev=21810
BUG: Fixed the crash reported in #0002992 (Crash after deleting a node registered as an incoming/outcoming node). The deleted node still remains in the I/O configuration.
BUG: Fix the issue that the incoming/outgoing nodes in the I/O configuration are not removed properly whe those nodes are removed from the scene.
Merge pull request #1 from tokjun/master

Fix the issue #0002992 reported in Slicer Issue tracker.