Add support for multiple includes in nested include

Currently this doesn't work:

````coffeescript
App.OperatorItemUser.create user: "komola", (err, user) =>
  App.Operator.includes([
    {model: App.OperatorItem, includes: [
      {association: "OperatorItemStatus"},
      App.OperatorItemUser
    ]}
  ]).all (err, operators) =>
```

Only OperatorItemUser would be included.