Break up the config

[rosshadden]

Right now, we require just "host":

{
    "adapter": "mongodb",
    "mongodb" : {
        "host": "mongodb://localhost:27017/sixtyvocab"
    }
}

We should do this instead:

{
    "adapter": "mongodb",
    "mongodb" : {
        "host": "localhost",
        "port": 27017,
        "protocol": "mongodb",
        "database": "sixtyvocab"
    }
}

This way we can default the port and protocol (and host, though that may get confusing), so the config would be simple.