Remove Canny pruning, enable multithreading

Canny pruning didn't improve average case performance, instead it
only increased the variance. Remove it to simplify the code base.

Perform face detection in two threads to make use of the dual core
CPU in WP8 devices.

Attempt to speed up integral image generation with a prefetch
intrinsic.
Implement real-time face recognition

Run face detection as quickly as possible and use the latest camera
preview data for every iteration. Keep track of the rectangles generated
by the face detector. Only schedule new recognition requests if a new
face appears.

Change the UI to reflect this. Overlay the first names of the detected
faces into the picture. Pressing a face will show more detailed profile
information.