Make Fluffle's image fingerprinting more resilient and extended

[Noppes]

Right now, Fluffle performs pretty badly when images which get reverse searched are slightly cropped. This is quite a common scenario, and therefore we should look into which algorithm offers more resiliency against cropping and other modifications.

Since this will require reindexing every image so far, we might as well add support for storing the output of a cryptographic hash function (like MD5). This would make it possible to reverse search the exact image. It should be looked into which hash function suits the best here.