Re-implements image listing to sort/filter on jumpgate itself

This should fix the issue with specifying a marker when listing images not working properly. This solution is not optimal since it sorts and filters client side.
What if limit was None here?  Now normally, does not make sense to pass marker parameter without a limit parameter but probably need a default here inside this array subset specification.  I.e.
sorted_images = sorted_images[:int(req.get_param('limit') or "20")]