ENH: VTK6 migration, address VTK6 library linking issues.
ENH: VTK6 migration, additional replacement of SetInput().

http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Replacement_of_SetInput
ENH: VTK6 migration, miscellaneous changes.
ENH: VTK6 migration, update vtkMRMLModelNode and sublcasses.
ENH: VTK6 migration, update vtkMRMLVolumeNode and its subclasses.
ENH: VTK6 migration, removal of update() from vtkDataObject.

Removal of Pipeline Update Methods from vtkDataObject.
http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Removal_of_Update.
COMP: VTK6 migration, use system png.

 To solve the "Incompatible libpng version in application and library", use system png as a hack.
COMP: VTK6 migration, a hack to slicer vtk library python wrapping.