ENH: Add SlicerExtension-VMTK extension

Description:
The Vascular Modeling Toolkit as a 3D Slicer4 extension.

Contributors:
Daniel Haehn (Boston Childrens Hospital), Luca Antiga (Orobix), Steve
Pieper (Isomics), Jean-Christophe Fillion-Robin (Kitware)
Merge pull request #665 from jcfr/add-SlicerExtension-VMTK-master

ENH: Add SlicerExtension-VMTK extension