Image feature detection

This pull request adds support for feature detection and image smartcropping into Wagtail.

It performs face/feature detection and crops images based on their content.
What's the use-case for passing a focal point to get_rendition (as opposed to using the image's own focal_point property)? It doesn't look like you're making use of this either here or in #477, and it seems to me that removing this would eliminate the need for the focal_point_key field on Rendition.