A demo of face detection&tracking

For face detection, Viola-Jones framework with a pre-trained cascade is used.
For face tracking, Kalman filter is used.

I believe that Kalman filter under-performs here, but it's hard to make it better with only 2 FPS. Unfortunately, the face detection engine I use here is written ineffectively, so it's about 5x slower than it can be.

I am not sure if I want to implement a faster engine (and improve Kalman filter quality when tracking the face speed would become feasible).

So, I just commit this as is.