Make internal keystore/pubsub system compatible with binary data

[DangerOnTheRanger]

As of right now, the keystore/pubsub system in storestub.py sends messages via JSON, which means binary data cannot be stored/transferred. This prevents #152 from getting implemented, since there will need to be a way to transfer video thumbnails back and forth when Redis is not used on certain Maniwani installations (such as when running in single-container mode). The easiest fix would be to replace the usage of JSON with pickling, I believe.