ENH: VTK6 migration, add vtk6 into build system.

WIP: Added vtk6 to build system and give users a build option between vtk5 and vtk6.
TODO: merge VTK6 release into slicer/vtk github repository and change the url in External_VTKv6.cmake.
ENH: VTK6 migration, replace SetInput().

Replacement of SetInput() with SetInputData() and SetInputConnection()
http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Replacement_of_SetInput