Model.extendsTo()

Usage:

```js
var Person = db.define("person", {
    name : String
});
var PersonAddress = Person.extendsTo("address", {
    street : String,
    number : Number
});

Person.get(1, function (err, John) {
    John.setAddress(new PersonAddress({
    }), function (err) {
        /*
          there should be a table 'person' (id, name)
          and a table 'person_address' (person_id, street, number)
         */
    });
});
```

Todo:

- [x] Basic usage working
- [x] Tests passing
- [x] Documentation
- [x] Extension model synching / droping