allow content authors to upload images

@smn this is a hash of [the security key](https://github.com/thumbor/thumbor/wiki/Security#url-tampering) and the image url to prevent DOS attacks. When saving an image, this gets automatically generated for detail image. It can be repoduced like so:
```
>>> from libthumbor import CryptoURL
>>> from django.conf import settings
>>> crypto = CryptoURL(key=settings.THUMBOR_SECURITY_KEY)
>>> crypto.generate(image_url='b0b4f5b505934d53a7abcbd3a135b912')
'/0aFRSCCoFZ_B082uzLhVhldCiB4=/b0b4f5b505934d53a7abcbd3a135b912'
```
and it can be used to generate a `300x200` thumbnail like so
```
>>> crypto.generate(width=300, height=200, image_url='b0b4f5b505934d53a7abcbd3a135b912')
'/zJHL5m_jsN9f01gWrE332Gw-Q8Q=/300x200/b0b4f5b505934d53a7abcbd3a135b912'
```