ENH: VTK6 migration, removal of manipulating update extent.

Removal of Methods for Manipulating Update Extent.
http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Removal_of_Methods_for_Manipulating_Update_Extent.
http://vtk.org/Wiki/VTK/VTK6/Migration/WikiExamples#Improve.
ENH: VTK6 migration, changes to scalars manipulation functions

http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Changes_to_Scalars_Manipulation_Functions.
http://vtk.org/Wiki/VTK/VTK6/Migration/WikiExamples#Improve.
ENH: VTK6 migration, removal of UpdateInformation() from vtkDataObject.

Removal of Pipeline Update Methods from vtkDataObject.
http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Removal_of_Update
ENH: VTK6 migration, replacement of AddInput().

http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Replacement_of_SetInput.
ENH: VTK6 migration, additional changes to build system.

http://www.vtk.org/Wiki/VTK/Build_System_Migration
ENH: VTK6 migration, use system zlib.
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, vtk library python wrapping.
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.
ENH: MRMLVolumeNode additional changes.
ENH: MRMLModelNode additional changes.
COMP: Support VTK6 in CTK tests

1) Support VTK6 in CTK tests
2) use SYSTEM_ZLIB
ENH: ImageData to ImageDataPort changes in vtkMRMLDisplayNode
ENH: TextureImageData API
ENH: Fixing blending