add nested object support

This adds nested object support, as in https://github.com/dominictarr/level-manifest/issues/1

For a db with:

```js
typeof db.nested == 'object'
typeof db.nested.get == 'function'
db.methods == {
  nested: {type: 'object', methods: {get: 'async'}}
}
```

the manifest is:

```js
manifest(db, true) == {
  nested: {type: 'object', methods: {get: 'async'}}
}
```

I'll be working on a patch for multilevel to support this too.