[WIP] Generation of images on the fly from URLs

This pull request makes it possible to generate images on the fly with urls like ```http://mysite.com/images/100/width-800/```.

Format of the URL: ```/images/<image id>/<filter spec>/```

The filter spec paramter is the exact same used in the ```{% image %}``` tag described here: http://wagtail.readthedocs.org/en/latest/building_your_site/frontenddevelopers.html#images-tag