ENH: VTK6 migration, add vtk6 into build system.

Added vtk6 to build system and give users a build option between vtk5 and vtk6.
ENH: VTK6 migration, replacement of SetInput().

Replacement of SetInput() with SetInputData() and SetInputConnection(), the same kind of functions include SetInput1(), SetInput2() and SetSource().
http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Replacement_of_SetInput
ENH: VTK6 migration, removal of GetProducerPort().

1)Removal of GetProducerPort() from vtkDataObject.
http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Removal_of_GetProducerPort
ENH: VTK6 migration, removal of Execute, vtkType deprecations.

1) vtkType deprecations
http://www.vtk.org/Wiki/VTK/VTK_6_Migration/vtkType_deprecations
2) Removal of Execute
http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Removal_of_Execute
ENH: VTK6 migration, removal of SetWholeExtent and update extent methods.

1) Removal of SetWholeExtent() from vtkDataObject
http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Removal_of_SetWholeExtent
2) WIP: Removal of Methods for Manipulating Update Extent
http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Removal_of_Methods_for_Manipulating_Update_Extent
ENH: VTK6 migration, change to AllocateOutputData.

Change to AllocateOutputData() in vtkImageAlgorithm.
http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Change_to_AllocateOutputData