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.
It's hard to read in this diff, but the list is not limited at all if limit isn't passed:

```python
if req.get_param('limit'):
    sorted_images = sorted_images[:int(req.get_param('limit'))]
```

But yeah, having a default limit might be good