Add AssociatedFieldsValidator

I needed this and I thought I'd add it here, in case you think it's a good fit:

```coffee
class MyApp.Child extends Batman.Model
  @validate 'name', presence: true

class MyApp.Parent extends Batman.Model 
  @hasMany 'children'
  @validate 'children', associatedFields: true
```

Will validate the children and add errors to the parent with the field names from the child. It's good for forms because sometimes a user won't understand an error like `"children is not valid"`. Do you think it's good for the framework?