Fix TVTK for VTK 6
Fix TVTK for VTK 6

The problem was that VTK 6 seems to return NotImplementedError for abstract
classes. So we just catch this exception and treat it as TypeError for VTK 5.

Fixes #72.

dmsurti/mayavi,dmsurti/mayavi,alexandreleroux/mayavi,alexandreleroux/mayavi,liulion/mayavi,liulion/mayavi