nested models

hey, i stumbled on `modeling` and i like how it looks. :)

is there a way to use a model in another model?

possible syntax that might already work:

```
var Address = Modeling("Address", {
street: {
label: 'Street',
description: 'The name of the street',
type: 'string',
rules: [
['required']
],
},
...
});

var Person = Modeling("Person", {
address: {
type: Address,
},
...
});
```

if this does already work, can we add it to the documentation?