URLs for serving images on the frontend

This pull request makes it possible to generate images on the fly with urls like ```http://mysite.com/images/<security key>/100/width-800/```.

Format of the URL: ```/images/<security key>/<image id>/<filter spec>/```

The ``filter spec`` parameter is the exact same used in the ```{% image %}``` tag described here: http://wagtail.readthedocs.org/en/latest/building_your_site/frontenddevelopers.html#images-tag

TODO:
 - [x] Implement HMAC authentication (https://github.com/thumbor/thumbor/wiki/Security)
 - [x] URL generator in admin
 - [x] Set correct cache headers in serve view
 - [x] Tests
 - [x] Docs