Image cropper

## Rationale

Pattern for cropping images. Allow the user to crop images in the browser. Cropping coordinates are sent to the server through hidden input fields that are both generated and filled by the javascript. On the server, a product such as imagemagick can do the rest.

## Markup

A first **draft**:

    <img src="foo.jpg" alt="Foo" data-crop="preview-id: somePreviewImage; preview-height: 40; preview-width: 40; aspect-ratio: 1"  />