MongoDB Registry

Hi,

I'm hacking a new Ruby framework and figured I'd give DCell a look as it seems to be doing some of the 0MQ heavy-lifting I need to do. I'm using MongoDB for a couple of other reasons, so I figured I'd hack in a MongoDB Registry. 

It works fine for the Itchy/Scratchy demo, but likely breaks for anything harder. I'm going to be adding more features, but my Free Time is up for the night.

TODO: connection options, authentication options, error handling