vtk: Added head

Tested on 10.8.3 and 10.7.5, with/without python bindings, and with PyQt.

This will enable people to test the next version of VTK (VTK 6, which should be released soon), and to migrate their code, as some API has changed since VTK 5.

Here is the migration guide to help you modify your code : http://www.vtk.org/Wiki/VTK/VTK_6_Migration_Guide