Makes the API useful: it now returns the template transformation

- Tracker::process returns the 4x4 transformation of the template
  in the scene
- debug images are available as Tracker::_debug and only if
  CHILITRACK_DEBUG is defined (automatically set in debug mode)
- ie, all debugging code is now behind #ifdefs
- statistics are now fully optional
- the Tracker and Template constructors have been simplified (in
  particular, a default feature detector and matcher is automatically
  created if not specified).
Added a Minimal Working Example