Add blacklist functionality

From time to time, our business is legally compelled to remove all traces of hosting certain content from our servers. We can delete the original assets and invalidate CDN links, but Thumbor will keep serving stored results until they expire, which is too long for our needs. To solve this, this pull request adds a new blacklist endpoint to ensure Thumbor never serves images from a blacklisted source file. It does this by checking any processed image source against the blacklist, and 404s if appropriate.

The blacklist can be seen at `/blacklist`. To add `image_url` to the blacklist, just PUT to `/blacklist?image_url`. The blacklist is persisted using your STORAGE engine of choice - without a storage backend, it won't work.

If merged, I'd be happy to add a Wiki page about the new functionality.