Multilevel Support

It's time to add multilevel support into LevelGraph, so we can query a remote database from the web and/or other sources.

As far as I get it, we should just add:
```javascript
db.methods = {
  graph: {
    type: 'object', 
    methods: {
      get: {type: 'async'},
      getStream: {type: 'readable'}
      //... all LevelGraph methods
    }
  }
}
```

And everything should 'just work': it's standard [multilevel](https://github.com/juliangruber/multilevel) + [level-manifest](https://github.com/dominictarr/level-manifest).