Changed url format, uses querystring for arguments

Not sure whether you'll want to accept this as it's quite a drastic change but I wrote it after noticing that the Heroku RedisToGo urls that I was getting were `redis://redistogo:asfk234@jack.redistogo.com:10364/` - notice no db number is specified.

Since it's impossible to write a regex to match the `redis:///path/to/sock/db/prefix` format and since the db and prefix are actually arguments (even the Django docs call them that), I thought they'd be better as query parameters.