Add VMTKSlicerExtension extension

This extension bundles the Vascular Modeling Toolkit libraries (http://vmtk.org) and provides the following modules:

1. LibraryModule - a gui-less module registering the VMTK libraries and providing the VMTK classes in Python.
2. VesselnessFiltering - a Python module providing Frangi's Vesselness filter
3. LevelSetSegmentation - interactive segmentation using Fast Marching initialization and either Curves or Geodesic evolution
4. CenterlineComputation - network extraction and centerline computation of segmented tubular structures

All modules are based on the Slicer3 extension but were enhanced in terms of usability and speed.
Merge pull request #279 from haehn/add-VMTKSlicerExtension

Add VMTKSlicerExtension extension