Integrate PRO multi-size support with Drupal image derivatives.

The Juicebox javascript library people recently released v1.4, which includes a feature they call "multi-size" image support. This appears to be a library-specific implementation of adaptive/fluid concepts that allows different source images to be delivered depending on the client's current viewport. It's in many ways their own native support of the concept explored in #2023851: Identify best Drupal tool(s) to use for adaptive/fluid image source generation with Juicebox.

This is a Pro only feature, which is too bad. In order to keep things as consistent as possible for all users I have not been adding too many Pro-specific controls to this module's GUI (PRO conf option can still be entered via custom key=value pairs, but their aren't nice form options for them). However, given the way that Drupal image styles should be able to integration directly with this new feature, I think this is an exception.

Adding support for this will probably involve adding some global conf options where PRO users can map a Drupal image style to the small, medium and large Juicebox screen modes, and then altering the XML build methods to set the needed largeImageURL and smallImageURL tags in the XML.

Related ref from their forum: http://juicebox.net/forum/viewtopic.php?id=1317