Face detection functionality

[Fshamri]

This is great but lacks face detection functionality, I hope you can do this feature to this fantastic library.

Thanks

[mosbth]

I assume that the main usage of face detection would be to automatically crop an image based on one or several faces detected on the image. Another usage could be to find and blur faces. But the basic idea is to find the x and y of the face(s) in the image, and then somehow use that info to process the image.

I guess that one option would be to use OpenCV as base for finding the faces. Another option would be to use some PHP port, there are a few smaller ones out there, for example mauricesvay/php-facedetection.

This is related to #59 which also speaks about "smart cropping" by edge detection and entropy.

[mosbth]

Another usage where face detection is useful is for automatic red-eye removal.