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.
Fix an import error

See the previous commit for details.
Merge pull request #73 from certik/vtk6_fix

Fix TVTK for VTK 6