Tune the mask used by goodFeaturesToTrack
Moved the detection of the eyes to FaceDetector

This is useful to remove false positives when detecting faces, and
will lead to the face distance estimation
Keep detected faces only if eyes are found as well
Fix issues with handling of LOST humans

Still not ok...
Use 'relaxed' eye detection, allowing for detecting faces further away
First attempt at estimating the human pose

Not very convincing yet...

A simpler/hackish version could be much better:
- compute the pitch and yaw of the face relative to the camera axis
- compute X,Y,Z in polar coordinates, assuming a distance r that
  depends on the intra-pupil distance.