Introduce memory-engine to mongo-like querying for fields of a subdocument using dot notation

So you can do:

```
model.find({ 'twitter.id': 1 }, …)
```

To find all documents that have a subdocument of `twitter` with a field of key `id` and value `1`. [Example](http://jsbin.com/umeros/20/edit).

Hopefully I've done everything right, but you might want to clean things up a bit.