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.
Built TubeTK in Prometheus

TubeTK is build as another external project of 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 'TubeTKinPrometheus'

* TubeTKinPrometheus:
  Built TubeTK in Prometheus