Repo cleanup for Mongoid 3.1.x

I was thinking of trying to flesh out your API a bit so that queries could be performed directly against the class of models for embedded documents. I.E.:

```ruby
embedded = EmbeddedModel.where('pos.gt' => 1)
```

instead of:

```ruby
root = RootModel.where('embedded_models.pos.gt' => 1)
embedded = root.embedded_models[0].in_collection.where('pos.gt' => 1)
```

Do you have any thoughts on the best way to implement that API? 

In the meantime, I cleaned up things a bit to bring it up to date with Ruby 1.9 and Mongoid 3.1.x

Great idea by the way, being able to query embedded models adds a lot.