ENH: Support VTK6.

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

2) Refactor functions in vtkMRMLVolumeNode and its subclasses.
Use ImageDataConnection instead of ImageData
Merge pull request #18 from finetjul/add-vtk6-support

ENH: Support VTK6.