Added a new URL formatter for image fields

A new URL formatter has been added for Image fields. This formatter has the option to display the (root-relative) URL to either the original image or an image style version of the image. This enables Views using a REST export display to provide the URL to either the full image or a resized (image style) or otherwise modified image. It will reduce the need for image processing on the client side and allow REST clients to use image style configuration from Drupal.