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.
Update VMTKSlicerExtension extension

Use git:// protocol.
Update VMTKSlicerExtension extension.

Fix a typo in the SCM url.