Added a way to get the raw connection from RedisDB

When attempting to write a RedisDB authentication snaplet, I ran into trouble because there's no way for me to run any redis commands from RedisDB in the IO monad.

Consider http://hackage.haskell.org/packages/archive/snap/0.12.0/doc/html/Snap-Snaplet-Auth.html#t:IAuthBackend .

I would like to initialize a redis db snaplet, then pass the connection it creates to the redis authentication snaplet. This patch provides such a mechanism.

If there's a better way to do this, let me know.