Also, I'm more comfortable having "dependencies" like `pymongo` and `pyzmq` in `requirements-dev.txt`. If I'm just writing a service that interacts with Redis using diesel, I'd rather not install `pymongo`, `pyzmq`, etc. I realize that we probably install some other dependencies (for some HTTP stuff) that might not be necessary in that use case either, but I'd rather not see the dependency list continue to grow for strictly optional protocol support.