Allows to save models in custom directory within bundle structure.

In <bundle-dir>/config/mandango/schema.yml:

```yaml
Model\AcmeDemoBundle\Foo:
    collection: <collection>
    fields: <fields>
    model_dir: Mandango
    Acme\DemoBundle\Mandango\Foo:
        Model\AcmeDemoBundle\Base\Foo
```

The new option is *model_dir*.

After generation files are saved into:

- Acme\DemoBundle\Mandango\Foo
- Acme\DemoBundle\Mandango\FooRepository
- Acme\DemoBundle\Mandango\FooQuery

This allows for a bundle to support both Doctrine backend and Mandango i.e.

In `Acme\DemoBundle\Mandango\Foo`:

```php
namespace Acme\DemoBundle\Mandango;

use Acme\DemoBundle\Model\FooInterface;

abstract class Foo extends \Model\AcmeDemoBundle\Base\Foo implements FooInterface
{
}
```

However, in `Acme\DemoBundle\Model\Foo` you have your regular model with setters/getters.