Face detection in lead image.

A few notes so far:
- on my test device (Galaxy S4), detection takes between 0.5 and 3
  seconds. May end up being too slow on other devices.
- fortunately, this will be easy to disable/reenable if necessary.

TODO: research a better way of passing the bitmap from Picasso directly to
the FaceDetector. Right now, we have to create an intermediate bitmap,
because the FaceDetector requires a 5-6-5 bitmap to work properly.

Change-Id: Ic255a0974345270c10164df84508527b6f2767fc