add new feature: face center crop

New feature:
Use specific face detector (or by default, Android face detector) to crop an image that contain faces and remain the ratio specified by {@link #resizeDimen(int, int)} rather than distorting the aspect ratio.

Add faceCenterCrop API, supporting cropping according to face
E.g.,
Picasso.with(context)
       .load(url)
       .resize(targetWidth, targetHeight)
       .faceCenterCrop()
       .into(imageView)
or
Picasso.with(context)
       .load(url)
       .resize(targetWidth, targetHeight)
       .faceCenterCrop(FaceDetector faceDetector)
       .into(imageView)