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.
Can we make the width/height fields nullable? It would seem appropriate for them to be null whenever x/y are null, and that way we sidestep the problem of old migrations / direct SQL code not knowing about those fields, that [we saw with Page.expired](http://wagtail.readthedocs.org/en/latest/releases/0.4.html#addition-of-expired-column-may-break-old-data-migrations-involving-pages).