Initialize repository

The structure of the application is intented for users to be able to
easily create a slicer-based custom application with little renaming while
keeping a source repository clean.
Add TubeTK as external project

TubeTK is built by Slicer. None of its modules
are built with it. Their build is delegated to Slicer which add them as
Slicer_EXTENSION_SOURCE_DIRS.
Merge branch 'external-tubetk'

# By Johan Andruejol
* external-tubetk:
  Add TubeTK as external project