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.
I think the setting should be named `WAGTAILIMAGES_FEATURE_DETECTION_ENABLED`. For someone browsing through a settings file, it's not clear what "feature detection" might mean (features of Wagtail? Browser capabilities? Featured articles?) without something to say that it's referring to images.